Round and round it goes, where it stops nobody knows! Ladies and gentlemen! We proudly present the mesmerizing, the death-defying, the still-functioning cerebral cortex of Mr Robert Downey Jr.!

So, he's sprung, sober and all settled down with a nice Jewish girl. But don't trip. America's clown prince, Robert Downey Jr., is still crazy - and brilliant - after all these years.

Our adventure begins with love. Old-fashioned, ring-the-bells, don't-wake-me-till-it's-over amore, which gives purpose to Downey's every move these days. His cell phone rings Crazy Little Thing Called Love. The man even sports a tattoo on his left triceps that says Suzie Q. Twelve months into their marriage, Downey and movie producer Susan Levin are still what he calls "boom-bah banga-roobah in monogamous deeelight." Which both tickles and astonishes him, he says. "Just before I met her, I'd decided I'd be a bachelor forever. It was very, very liberating. I thought I'd simply sport - f*** my way across the continental United States and then get on a slow boat or something."

Instead, Suzie Q got a stomachache on the set of Gothika while producing the 2003 psychological thriller starring Downey and Halle Berry. Downey stepped in to play shaman, which is how Downey and Levin got together. "I'm quite well-versed in the area of tinctures and herbal remedies," he says, unzippering a small man-bag over lunch in Brentwood to reveal no less than 30 little green capsules - "my daily arsenal of vitamins," he says, pronouncing vitamins the British way - and two single-ounce vials of a liquid labeled SFX Cxvrm, which Downey says is for parasites. 'We were just on vacation in Hawaii, and a couple days before we came home, I sunk into this chasm of dread about returning to the real world. Then we get back and... long story short, we did some electrodermal screening where you plug in and figure out the thousand things you're allergic to, and I see I've got parasites clinging to my lungs, which explained everything. For me, if something's off by 2 to 18 percent, my whole world is collapsing."

It's all about maintaining the levels or, as Downey, dressed in distressed jeans and a Mötley Crüe concert shirt, puts it, "making sure the engine fluids are all topped off." Serotonin. Testosterone. Caffeine. Nicotine. ”And after two years of being on this gastronomic experiment that brought me to near starvation, I realize complex carbohydrates are essential, too. For steadying the headlights."

One would think this might be an ideal moment to segue into a conversation regarding Downey's latest movie, A Scanner Darkly, based on the Philip K. Dick futuristic drug novel about the effects of various chemicals on the human brain, but Downey doesn't really want to go there. At least not in the usual actorly way of talking up his "generous" costars (in this case, a Hollywood rogue's gallery that includes Winona Ryder, Woody Harrelson and Keanu Reeves) or his genius - absolutely genius - director (here, Richard Linklater). No, Downey's idea of hyping a movie goes like this:

"The impulse to alter consciousness," he says, "is very old and very important - starchier pasta here, sugarier water there. We need the fix. So for whatever residual deficiencies are resultant from what, in my case, was pretty serious drug and alcohol abuse," and he's talking here about suspected memory loss, continuing financial headaches and the occasional screaming nightmare, "I can't say I would undo all those experiences. Nothing beats dropping a near-lethal dose of mushrooms and going to Idaho to your friends ranch. It's the greatest..."

Bear with him.

"...because here's the truth. I'm reputed to be the poster boy for insaniac, drug-fueled ne're-do-wellism, and I can't deny any of that, but past a certain point, it wasn't much of a choice. At some point, you just find yourself in the backwoods of Pennsylvania wondering whether your runaway train will crash, and whether it does or not only has to do with luck and chance and physics."

This strikes us as an excellent time to call in the authorities. Linklater says of Downey, "Robert is his own unique specimen," as if we needed confirmation. "One human body is almost not a large enough vessel for all he contains. His mind is so rich with so many thoughts about so many things and ideas. That's why acting is a really good outlet for him."

On the set of A Scanner Darkly - and yes, the movie did have a set even though the filmmakers relied on the drawing-over-live-action animation technique known as interpolated rotoscoping, which Linklater used in his 2001 movie Waking Life - there wasn't a whiff of party atmosphere, which is perhaps surprising given the players and subject matter. Says Linklater, "The vibe, certainly where Robert was concerned, was one of pilates and yoga and martial arts."

Then there is the broadway musical. Downey has been composing it in bits and snatches since his early 20s, when he was primed for stardom as the newest cast member on Saturday Nigbt Live, which he joined in 1985. That lasted all of one season. He went on to film roles as addicts and rich kids, but all was forgotten when he donned the Little Tramp gear for Chaplin, a performance that got him an Oscar nomination. The destruction, however, was already well underway. Downey's father, Robert Sr., an independent film director, famously offered him his first hit of pot as an adolescent. Downey Jr. has characterized his Manhattan youth - his mother, Elsie, acted in dads oddball movies - as a "boheme pressurefest."

The bad years are past him now, he insists. The marijuana, which led to booze, which gave way to heroin, coke and three very high-profile arrests followed by court-ordered rehab - it's practically biblical history. Downey survived, and that's the main thing. Fortunately, I was a street kid and made it through a lot of dangerous situations growing up," he says, when asked about the lingering impact of his incarceration for violating parole ten years ago, "so it wasn't like I got there as some trust fund or oil magnate's kid who squirmed around going, 'Oooo dear, there's this f***ing enormous cockroach in my cell.' Still, it sucked, you know?"

And now, even with the love and the herbs and the therapy and the yoga and the regular meetings he can't really talk about (From what I've heard, they say it's an anonymous program," he says. "But I imagine if someone were to do a program like that, they should do it frequently or not do it at all,") the struggle is real and it is daily. As Downey says, "I'm aware of what I can't do. I'm not gonna go on location and do some film-noirish indie movie in Bulgaria for 12 weeks. Not because Bulgaria's not a great place. It's because, in Bulgaria, prostitution and narcotics trafficking and that type stuff is available, and for now, that wouldn't be OK."

And so... the musical. Why not? He could do a lot worse for himself. Singing, dancing, plus, it's a real uplifter: It's the last of four seasons of life on earth as we know it. End times, life going down the tubes, final winter. Downey hasn't worked out all the scenes, but he knows it will feature a modern-day Cupid who screws up by shooting himself with his own arrow and falls in love. Cupid needs to rectify all the worlds problems before the day is through.

There's also a kid involved. Downey has a son himself. Indio, whose mother is singer-actress Deborah Falconer, Downey's ex, is 12, and handsome, forgiving. "We have a lot of contact now," Downey says, "which I love." But in the musical, the kid is older, not quite a true representation. He's a college student in Philosophy 101 who's tired of his teacher's verbal diarrhea and sees no reason why he can't find enlightenment right here, right now.

"The world is ending, but people haven't given up," Downey says, and you know, the play might be horrendous, but you kinda want to see it anyway. Nothing kills the hope that all the acts of human failure and fragility will somehow be made right again before it's too late."

Downey stops and looks away. The vibe has suddenly skirted a little too close to something that feels absolutely genuine, and he realizes he's late for his Wing Chun kung-fu class, which he describes as "you and one guy whose name is Eric, but you would never call him Eric because he's your f*****g sifu, and he can kill you, and that's the end of it."

The actor zips up his man-bag and pops his last of three daily Camel unfiltereds between his lips. Downey takes a long drag, long enough to open the door for one last probing inquiry: Are you happy these days?

He lets loose a stream of gray smoke and does a long yoga stretch, his vessel expanding to contain just a little more.

"Listen," he says. I'll talk to people who are f*****g up, who are on the brink, who are where I was. And I'll say something to them now. I'll call them on things. I'll say, 'I see you on the other side of this.' Half the time I'll say it 'cause you're supposed to say it, but then sometimes, every once in a while, you see the light go back on in homey's eyes and you're like, 'Holy s***, maybe there's a reason I've been put on this Earth."

Now there's something to write a musical about.