"You know what? If you take away all the crap in my past, I really am a nerd, a normal guy. Ask my fiancée. I sit around and watch the history channel, I go to the gym. Or ask my son. He knows I'm a nerd, and such a goon."

His answer was in response to my question about Gothika, and how unusual it is to see him playing the sane person as opposed to the kooky one. He is, indeed, the levelling influence in the film, a convoluted and mostly effective psychological thriller about the not-quite-dead and the reborn - areas with which he can readily identify.

He grabs himself a glass, pours in a little water and has instantly made himself a makeshift ashtray. There is none of that dance in which the perceived subordinate (the interviewer) must furnish the star with the facility for ashing his cigarette. This is one low-maintenance nerd.

Just over a decade ago, a 26-year-old Downey Jr. had Hollywood and moviegoers alike enthralled with his eerily familiar portrayal of Charlie Chaplin. The role earned him an Oscar nomination, and despite it being probably the best male performance of the year, he didn't win. We Brits, more inclined to reward a performance than a body of work, handed him a Bafta as a consolation prize. He did not reap such rabid critical acclaim again until 2001, when he deservedly won an Emmy and a Golden Globe for his guest role in Ally McBeal, having given the ailing series a serious kick up its lethargic 'ass', as they say in America.

Hollywood is replete with suits and stars happy to reiterate the fact that Downey Jr. is one of the greatest actors alive. Ask him how he feels about the compliments and he shrugs his shoulders and does his best impersonation of Embarrassed Nerd. "I so don't care about that because I don't know, and have never known, how to respond to that. If you know what you know, are you going to stop a great football player mid-play and say, 'Boy, you certainly know how to wide-receive'? He'd be like, 'Do you mind?' But saying to him, 'Boy, that coach has really helped you get better' and 'That gal gives you reason to run faster', well, that's what it's all about."

And besides, few of those happy to eulogise Downey as an exceptional talent actually thrust contracts at him when he was once again free to work, usually citing insurance problems as an excuse. Certainly, Hollywood bonding companies are notoriously skittish about hiring actors with as unpredictable a history as Downey Jr's, but anyone who really wants him can find a way.

Joel Silver, Gothika's bullish producer, did. Woody Allen didn't, and the latter's failure to follow through on the offer of a starring role in his Melinda and Melinda (to be released later this year) still rankles with Downey Jr. "It's not that they couldn't get it. It's that they did the best they could, but they waited several weeks until they started to inquire. So there was no way. On Gothika, they figured it out before they offered me the job."

Mel Gibson, on the other hand, signed up his old friend for his first post-release job in The Singing Detective, which failed to scale the critical or commercial heights set by the seminal Dennis Potter TV series. That Gibson is close friends with Lethal Weapon producer Joel Silver, who provided Downey Jr.'s second job, is surely not coincidental. Like The Singing Detective, Gothika has not made significant waves in the US, but the combination of the two has imbued Downey Jr. with a tentative new campaign slogan, albeit one that's unlikely to trip off his tongue: ‘He's back.’

It's just that he's not at all sure how he feels about such assertions. "All I can say is that every medicine has a side-effect. I've been studying Chinese medicine and meridian therapy [also Chinese in origin, which utilises chi meditation to ward off disease and addictions]. And I didn't realise it before, but the 'medicine' I was doing before had a terrible side effect - you end up in jail. The side effect of being back and doing good work, and enjoying being back in life and society, is that you've got to work it, you've got to sell it. You don't really have any free time per se. It's Sunday lunchtime and isn't there something we would both rather be doing? Weren't you thinking, 'When is this self-obsessed jerk going to get in here so I can do my job? What do they think? That I exist to sit in a hallway waiting for them?’ But that's the deal. Everything has a side-effect. And if I start focusing on the irksome stuff, like doing 80 TV interviews yesterday, well, I'm back in that place of every time I feel I was wronged. But then I haven't really been wronged that much. I don't think any of us are wronged much in this life."

He would rather be with his ten-year-old son, Indio, whose upmarket LA private school is running a basketball clinic this afternoon.

Downey Jr. comes across as honest, funny, empathic, excellent company and endearing in every way, qualities rarely seen together in any of his colleagues. He is also pragmatic. "I don't see my life now through rose-coloured glasses, I’ll tell you that much. I think about these books I used to read when I’d go stoned to the Bodhi Tree (a famed hippy, new-age bookshop and oasis for stressed denizens of LA). I'd think, 'Wow, I'm so spiritual, reading all this stuff'. It turned out I was attempting to live my own philosophy, but really it was a bit like taking off on a quad-bike without reading the warnings."

Downey Jr. met Gibson in 1989, when the pair starred together in Air America, and has since had flashes of brilliance - with the exception of Chaplin, these have rarely been acknowledged - in films ranging from Short Cuts and Natural Born Killers to Curtis Hanson's superb Wonder Boys.

At some point during our conversation, Downey Jr.'s long-suffering publicist enters the room. "Oh, he's just my dealer," smirks Downey Jr., who is as unable to resist sending up his chemical dependencies as he is to blame anyone or anything for them.

"I wish I could say that I was too young to handle fame when it came, and that I'm more ready to handle it now. But I might be less able to handle everything in five years' time. I like to imagine that you build towards things as you grow and mature, but it's just like hotel management. There are so many different things you have to tend to, and every year they decide which hotels get five stars. It's not like they get stars and keep them forever. Apropos, the great thing about doing Chaplin in my mid-20s was that I can still pretend I'm an authority on someone who - no offence - is 12 times greater than anyone who is acting today."

Indeed, if anyone is a guiding light in Downey Jr.'s life - besides his son (by his ex-wife Deborah Falconer) and fiancée (Susan Levin, a producer of Gothika) - it is Chaplin. "He's a great example of someone who had every reason to fail in his adult life, but instead of blaming the present on the past - his difficult childhood and stuff - he made lemons, lemon trees and orchards. He basically reinvented the lemon, and is the greatest example I've ever seen of utilising everything you have in your artistry. I don't think there'll ever be anyone like him ever again."

Downey, meanwhile, permits himself only a modicum of self-flagellation while endeavouring to transform his own lemons into lemonade. "I do feel sad for certain things, about what I allowed myself to become. But not as sad as I'd be if I was here thinking, 'God, I can't wait till this is over so I can go and buy a bag of coke.' That's pathetic. What is it that's so awful about having the same opportunities as everyone else? Why is it so important how I feel, and why am I so dramatic that I need to medicate myself? What is that? Jesus Christ. What am I? Jean Harlow. It's just Robert here tapping his cigarette into a glass. Hey, I wish he was really deep and supernatural, like Gothika."

But no. Downey Jr. is, remember, just a nerd who loves that he has hooked up with someone who is not an actress. "I'm coming up in the world. Yeah! Female producers for the most part are not crazy. That's kind of cool. They have to manage productions. So, yes, this is a huge progress for me. I didn't nail any of the actresses."

But seriously? "Well, seriously, love is great therapy."