Yesterday, when we met for the first time - the 45-year-old actor throwing kung-fu pirouettes at me while squeezing my biceps and gauging the strength of my build - he promised a "restorative work-out. We're not going to kill you, duuuude." Sounding every bit the keenly protective father figure, Downey added reassuringly: "It's about health." Now, facing the grinning actor in his low-key private gym, the Santa Monica Body Building Center (the same sweat shed where Sylvester Stallone trained for Rocky), I feel like I've walked blindfolded into a carefully engineered bear trap for gung-ho journalists who fancy their chances at out-ironing the Iron Man.

"Now, this," Downey declares to the room, walking over to a vast structure of white painted steel that resembles a cross between a sex swing dreamt up by Philip K Dick and a medieval torture rack, "is what we call, 'the War Machine'." Looking at the spherical tangle of pulleys, weights, clips and twisted metal, I kick myself for picking out the fly-away Orlebar Brown running shorts and teeny-tiny wife beater this morning, rather than the chain-mail vest. Or the Hurt Locker-style bomb-disposal suit. "You've got some serious Chariots Of Fire duds going on there," beams Downey, effervescing wicked glee. The smile drops: "Wait till Jimmy shows you the sledgehammers."

Watching Downey pump it as if his life depends on every lift, it's clear he works on pure instinct. Gwyneth Paltrow, Oscar winner and Downey's love interest, Pepper Potts, in the Iron Man movies, concurs: "Robert's greatest skill as an actor is his versatility; his ability to play many things at one time: incredulity with an undercurrent of self-referential humour, pathos with warmth, triumph with a hint of frailty; whatever the combination, there are always many levels to what he is conveying. And he can do it in any accent." As for the ride Downey provides for those working with him on set, she adds: "I don't think I have ever seen Robert stick to a script; he is the most fly-by-his-seat-actor I have ever worked with."

Dressed to sweat in black jogging bottoms, a cherry-red T-shirt, elbow guards, black trainers and a black, woolly beanie, this particular morning Downey may look like a well-rehearsed gym bunny, but out of the three other times I encounter him over these two days, his outfits never stray too far from "comfy". He seems to dress like a reserve astronaut perennially on stand-down due to bad weather.

As he squats and sips an energy drink in the corner of the gym, an old war wound on his shoulder being strapped up by Jimmy, his Irish-American bodyguard, while his trainer of five years explains to me, somewhat unconvincingly, how Downey's work-outs are about "partnership" rather than "punishment", I notice there's something uncannily simian about Downey's physicality. Perhaps that's why he prefers such soft, stretchy elastic cotton clothes. They give him room to roll, to bank, to bounce, to coil, to remain fluid - to stay on high alert. Downey is, and has always has been, about energy - physical, contained, unleashed or otherwise. The wattage coming off the guy makes the other bodies in the room - even the unit that is Jimmy, who's inked-up like a Manhattan-bound A-train, an Iron Man motif down his left calf, the digits 221b on the underside of his forearm (how's that for devotion?) - orbit him like spare moons.

"I'm told that I am getting bored less easily," he explains when I ask him whether all this huffing and puffing is absolutely necessary. "I'm managing my impulse to scream, 'boring alert!' a little better. 'Boredom is self-obsession' - I don't know who said that but I think it applies. So am I becoming less obsessed with myself? I think as a function of age you have to, otherwise you can't stay healthy."

The work-out is tough. Not throw-up-all-over-myself-and-fall-back-out-into-the-street-whimpering-to-mummy tough, but hard enough to cause Downey some minor concern. "You OK?" he asks sporadically between swinging 15kg iron kettle bells through his legs (OK, 10kg for me). Gasping, I lie - you have to, right? - and tell Downey I'm "fine", blaming the double espresso with half-and-half I had earlier, but even Jimmy can see I'm feeling pain in muscles I never even knew I had until today.

"Don't worry, British," consoles Downey as we jump into his dealership-pristine ice-white VW Golf, turn down Howard Stern and slipstream into the Venice Beach traffic, just as if he's one of the hundreds of thousands normal Los Angelenos going about their working day, "I've got the cure."

Rebuilt, reborn and restored, Robert Downey Jr has Hollywood at his feet - again - and this time he swears there's to be no reappearance of "Retread Fred, the serial relapser", the term coined by Downey himself to describe his inability to stay off a $4 crack pipe and keep his sorry ass out of a prison-orange jumpsuit. Since director Jon Favreau cast him in Iron Man in 2007 - for which Downey did a screen test for only the second time in his career - Downey's Hollywood credit rating has blown through the roof, smashed the billion-dollar weather balloon at the box office and rocketed into deep space: current position, 45,350 miles past Pluto and climbing. This isn't a comeback. This is a full-scale, karma-reversing body swap.

The three-storey frosted glass and concrete house he takes me to is a monument to Downey Version 4.5. Before I'm shown "the break room" (basically a guest bedroom where I'll shower and change), Downey gives me a tour. "First floor, business": this is the office space where six or so members of Team Downey, the star's production company set up with his wife Susan in 2010 with a first-look deal at Warner Bros, keep a handle on business, which at the moment includes filming Sherlock Holmes 2 in London with Guy Ritchie and Jude Law, pre-production for Yucatan - a heist movie set in Mexico originally devised and worked on by Steve McQueen prior to his death - and looking for a movie for Downey to direct.

"Second floor, pleasure": along with the break room, this is where the Downeys sleep and - well, he said it - pleasure themselves. Their bedroom is a dark affair, the Venetian blinds pulled down with various Conan Doyle books on a dresser, clean wooden surfaces stained the colour of black coffee and a low-riding bed with leather trim and matching bedside tables. The bathroom, with its mosaic tiles and straight-edged, free-standing bath big enough for two, is about as messy as a room can be without it being embarrassing. There's also a small balcony where Downey admits he likes to crouch down in the early evening, alone, and eavesdrop into people's conversations while they stand outside his front door.

"Third floor, sustenance. And access to fish tanks": the uppermost floor is where we'll have brunch, consisting of papaya and mango cubes to start, followed by red-pepper quiche and ending with a gluten-free coffee cake with frosted top, all cooked by his private chef. On this floor there is a square sunken seating area with various Taschen editions, several carved wooden insects, a free-standing piano - a gift from Susan - adorned with a Kerouac inscription from On The Road ("the only people for me are the mad ones") and a plastic Iron Man figurine doing the splits. There is also a rock garden and a terrace, presumably where Downey walks out on occasion, breathes deeply and laughs like a hairdryer into the face of all he surveys and into the narrow minds of all the industry naysayers who doubted his future couldn't be so goddamn sweet.

"Here you go, buddy." After a shower and a surreptitious peak into his son Indio's room (basically a holding cell for the teenager's growing guitar collection), I join Downey at our table for two on the third floor. "These should sort you out." Next to my chair are three packages: the first, a rucksack-sized packet of Epsom salts (weird, I say, as that's the sleepy suburban town where I was born; although I begin to notice weird Klingon mind-sifter moments like this happen around Downey all the time) and two small boxes of Arm & Hammer baking soda. What am I going to do with these? Bake a cake? "You have a bath back at your hotel?" he asks. I do. "Run warm water and empty everything into the bath and then get in. I wouldn't stay in there for any longer than ten minutes." Why not? Downey leans forward and taps the packet of salts in front of me. "Powerful laxative, dude."

Although the consummate host and a killer gym buddy, the reason we're both sitting here is for the release of Due Date, out this month. It's directed by Todd Phillips, the man responsible for The Hangover, a movie that this summer became the biggest-selling comedy DVD of all time. Although Due Date features a cameo from gnasher-flasher Jamie Foxx, the true co-star of this project is, unsurprisingly, Zach Galifianakis, the laugh-until-you-wet-yourself "reh-tard" from The Hangover; a man who's part grizzly hobo, part Curly from The Three Stooges.

Due Date is basically a buddy movie, or as Downey puts it, "like the modern, next logical step in a not-too-realistic-but-definitely-natural evolution of two schmucks on the road." Downey plays fast-talking, uptight rage queen Peter Highman, an architect who is jetting back from Atlanta to LAX so he can be at his wife's side at the birth of their first son - in a nutshell, the straight guy. Galifianakis plays Ethan Tremblay, a gay, out-of-work actor with an air-dog who's heading to California to be discovered, or as is more likely we discover, be forgotten and end up smoking his own weight in weed between bussing at the local Jack In The Box. The pair, their opposing lifestyles, opinions and hygiene habits crash headlong into each other at the airport, after which they are both escorted off the plane by an air marshall with a Taser and, now banned from American airspace, forced to make the 2,000-mile, journey together in a cramped, claustrophobic hire car. As is expected, carnage ensues, as Tremblay's chaotic, stoned manner and inability to stay awake at the wheel leaves Downey's character broke, busted, bruised and finally - this is Hollywood, remember - reborn. The movie is cut from a very similar comedic cloth as The Hangover - Phillips will do for early-thirties frat-boy humour what Judd Apatow did for twenty-something languid stoner bros.

Unusually for Downey, his role is to anchor the scene rather than blow its wheels off. It's an odd fit, especially considering Downey's last play-for-laughs part that had him black-up for the deliciously controversial role of Kirk Lazarus in Tropic Thunder, a part that still makes the actor, "uncomfortable, and weirdly apologetic". Here Galifianakis is the one allowed to get creative and punch an anarchic hole through the set.

"It's kind of embarrassing to say this," agrees Downey, "as I always thought of myself as being that type of an actor, but Zach is probably one of the freest artists I have ever experienced. The funny thing for me was having the confidence to play the straight man for once, and it gave me a whole new set of insights into the people whose responsibility it is to hold the mooring lines while the other great talents have chewed up the scenery - and I've been that guy."

Fatherhood and self-discovery are the big themes coursing through Due Date, themes that, one imagines, Downey has spent plenty of time in front of therapists discussing. The relationship between Robert Downey Sr. and his son is a significant one, especially when trying to decode Jr's maverick genome - containing the firecracker gene that made Downey "that guy" for so much of his career. Downey Sr. was a cultish, independent film-maker working in the Seventies and Eighties and was, in fact, the director to give Jr. his first line on film. The clip of the actor, aged five, on the set of Pound is uploaded onto YouTube for anyone who cares to see Iron Man pre-secondary school. The ultra low-budget flick is set in a dog pound and all the actors play one or other of the incarcerated mutts. Downey Jr, for his first time ever on screen, has to look straight at a badass bald guy playing a Mexican Hairless and utter the words, "Have any hair on your balls?" As far as first lines go, it's memorable.

His father's bohemian lifestyle, however, didn't exactly help provide a stable environment for a curious kid out to have as much fun as possible. Downey Lore has it that his father gave him his first spliff aged eight, although it was far more likely to have been one of his dad's rabble-rousing hangers-on. As Downey Jr. himself admits, "It was such a permissive time. And we weren't discouraged. As I remember it, I was swinging in a hammock one day, and there was a guy, one of my father's friends, in the room and I literally just put my hand out, and he walked over and gave it to me. And it was on."

Although small parts alongside Anthony Michael Hall in John Hughes' Weird Science came first, Downey Jr's big cinematic breakout came in 1987 with the adaptation of the Bret Easton Ellis novel Less Than Zero. Even back then he was tapping into that tumultuous relationship he had with his own father, using fragments of his own experiences and psyche to bolster his on-screen characters, something he still does to this day. At that time Downey had progressed beyond the odd hit of marijuana: "I took my drugs after work and on the weekends," he admits. "That changed on Less Than Zero."

Talking about one particularly gut-wrenching scene in which his character, Julian, goes back home to ask his father for help getting off drugs, Downey adds: "My life, personal and professional, I've always had these big themes to do with fatherhood and taking responsibility for my own actions. The first day of shooting Less Than Zero, I'm on a tennis court with the director Marek Kanievska, and I really have no idea what I'm doing. So I started thinking, 'Do father and sons ever really connect? Are they ever truly able to love each other and accept each other?' At this point, I'm getting really choked up... When the scene was done, Marek said, 'You know you're a real actor and I'm going to craft this whole movie around you. People are going to see who you are.'"

Talking to Downey can at times feel like being caught in an asteroid storm of Los Angeles psychobabble; a sort of deep-sea introspection full of boxes within boxes and questions locked within semi-confessional proclamations that may or may not be exaggerated. He's like a cross between a Russian doll and the Large Hadron Collider, a stream of ideas and atoms that smash into one another to reveal more dead-end answers and throw-away revelations. You could knock it as befuddled, sage-burning pseudo-hippie therapy talk if only its restorative effects hadn't harboured such incredible results for the man who, only ten years ago, started a lost weekend with a visit to a strip club, and then finished it two days later thanks to a tip-off from a anonymous caller who told police in reference to the strung-out star, "Uh, yeah, I'd just like to let you know that in Room 311 of the Merv Griffin there is a man that is doing an ounce of cocaine and with a couple of guns and is pretty upset."

The last time Downey would feel the cold bite of LAPD steel at his wrists would be on 24 April 2001 in Culver City. As the cop on duty, Yvette Countee, would later testify: "While I was speaking to him, I then noticed his speech was rapid. He interrupted me on several occasions and rambled on without any questions being addressed to him." Just four months previously Downey had picked up the Golden Globe for his role in Ally McBeal, and, although praised for boosting the show's ratings, this latest public run-in with the law proved too much for the series' producers - he was let go.

In court on 16 July 2001, Downey made a "no contest" plea to possession, as his urine sample had confirmed traces of cocaine. However, thanks to Proposition 36, a new law just passed in California, he was ordered to spend a year at a residential rehab facility, with three years' probation, rather than tie up another county jail bed. He was lucky. But now, finally, even Downey could sense he was playing chicken with his seemingly endless supply of serendipity.

Of course, it wasn't jail, endless botched trips to rehab or even the threat he might never work again that made Downey pack in the bad-boy behaviour. Instead, it was that old romantic - love. Downey Jr. met Susan Levin, then executive vice president of production at Silver Pictures, on set of Gothika in 2003. Although batting away the star's advances with sensible parries such as, "He's an actor, I have a real job," eventually Levin acquiesced and the pair began dating. Downey proposed to her the night before her 30th birthday and they married in New York in August 2005.

"I'm afraid no one keeps me in check - not even Susan," laughs Downey. "I can tell by the look in her eyes if I need to immediately stop a line of behaviour. Or whether she is somehow or other able to withstand the onslaught of wanton, well-informed narcissism that is coming at her like an über-tsunami." Do they both have to work at the relationship? "More than I thought. And usually more than I would care to. But ultimately, it's just so nice to have pure communication with, for me, a woman. Historically this has been something that, for me, was not really achievable."

Downey's first serious girlfriend, back when he was doing theatre in New York in the early Eighties, was Sarah Jessica Parker - later to be forever known as Carrie Bradshaw from Sex And The City. Devastatingly for Downey, she just couldn't handle his love for a party and the relationship blistered apart. In 1992, he met Deborah Falconer and within 42 days they were married, Deborah pregnant with Indio soon after. Then, as Downey explains, "The Nineties happened, and it was just about survival."

"Susan is cut from a slightly different cloth to Deb and I, and she's provided a great new framework for a lot of things. Susan doesn't really care if this building fell down and we never did another movie again so long as we were happy and loving and together, but so long as it hasn't she's operating on the fact there's a lot of shit to get done. She is a master at it - I wouldn't be surprised if she'd built the pyramids."

To this day, Downey doesn't believe in the rehabilitative effects of jail, considering its value more akin to containment. "I do think there is something divine about large-scale humiliation, but no, it never worked for me. It never produced enough of a blow back for me to catch the drift. It doesn't so much produce results as just get your attention; you become a fish in the barrel and there's really something quite beautiful about that. Jail didn't help me any more or less than anything else. But I now know I created that experience for a reason, and I'm only just starting to understand those reasons. Remember this is California, right? 'Come on vacation, leave on probation.' I mean, I can't believe I stayed out of the pen for as long as I did. And I didn't mind it inside really; it wasn't so bad."

I tell him I find this hard to believe. "Listen, a prison is just like a public state school. Did you ever go on a film set on location? It's just like prison. Have you ever been in a bad relationship? It's just like having a celly who wants to kill you. Have you ever been in a street fight? It's just like chow-time. Have you ever been to a rave-up? It's like a yard riot. I wouldn't wish it on anybody, but it neither fazed me, nor changed me, nor informed me - but maybe it toughened me up a little bit. It's kind of like a lazy military operation with great moments of delight and horror. Pray you avoid it."

To watch the actor, as i leave him now, discussing with a pretty young member of Team Downey who, out of the two of them, will be calling the star's mother-in-law to discuss the arrangements for a forthcoming Hawaiian family holiday, it's remarkable to think that all this - the three-storey modernist house, the two humdinger movie franchises, the action dolls, the silly-millions in the bank, the second wife who keeps him grounded (despite his protestations otherwise), the son who considers him a real-life superhero, the otherworldly introspection - so nearly didn't happen. Considering Downey's staggering acting talent, if it weren't so very nearly true, it would be laughably preposterous.

Before I leave, I ask Downey whether he ever begrudges the Hollywood elite for not honouring him with an Oscar for Chaplin in 1992. He was nominated for Best Actor but lost out to Al Pacino for his role in Scent Of A Woman. According to legend, as soon as Downey, then backstage, saw how many little gold men they were giving out that evening, his nervousness transformed instantly into flippancy at the idiocy of all the glamorous industry hoopla.

"No I wasn't hurt," he states without missing a beat, followed by a classic Downeyism. "First of all, I want to qualify that there is no physical pain by not winning something. There is psychological turmoil and lack of understanding, but what is really going on here? Because I can say that I've been stiffed. I can lie, and pretend I worry. But I don't care. I. Don't. Care. Honestly. There was a time when I thought the only way out of hell was to win a certain type of award. And then I thought, what a pitiful existence."

We walk over to the edge of his balcony where the iron, slate and concrete movie-star citadel meets real-world meandering traffic and the threat of a double-dip recession.

"Look, as long as I stick around I'm going to end up with a bunch of them anyway as they're going to run out of people to give them to. And I'm probably going to win it one year when someone else deserves to win it. Why? Because it's my time, goddamnit. And that's the way shit works around here. I'm just an uptight mutt at the top of his game. Welcome to Hollywood, bitch! I'll see you at the Vanity Fair party and I'll be holding that golden statue you deserve 'cause guess what? It happened to me too, motherf***er!"

And with that Robert Downey Jr. looks at me wide-eyed and laughs so hard he can hardly believe it himself.