Robert Downey Jnr: Marlon Brando on acid, anyone?

Robert Downey Jnr is supposed to be clean now, but seems far from serene, to paraphrase the old Betty Ford Clinic maxim. It's not that he's in a bad mood or anything, far from it in fact. At the luxurious Hotel Park City, in Park City, Utah, he's doing his publicity duties for his latest film, The Singing Detective, which has just premiered at the Sundance festival. Shepherded from journalist to journalist with his co-star, Robin Wright Penn, by the PRs, he answers questions dutifully enough, but the man simply won't sit still.

He hunches over the tables and looks intensely into your eyes. Then he leans so far back that you worry the chair might topple over, and he looks at the room. He roars or minces or trills when impersonating other people, and then has a whole new set of verbal tics and growls for reporting his own inner thoughts. At one point, apparently a bit antsy, and clearing gagging for a cigarette, he gets up and opens the window to let some air in, and paces around the table for extra measure.

Knowing everyone has been tiptoeing around his recent brushes with the law, which have resulted in several stretches in prison and rehab (his last arrest, for cocaine possession, was in April 2001), Downey himself brings up his travails before I even get the guts up to ask about them. He shows me his festival pass, a huge plastic folder slung around his neck in which he has also inserted his driving licence and a picture of his son Indio. "It'd be great if I had my booking shot in this, too," he jokes, referring to what's commonly known as a "mugshot". "I think you can get copies."

Clearly, he's feeling pretty buoyant about the film, which went over quite well at the previous night's premiere. Based on Dennis Potter's original seven-hour television series of 1986, this US feature-length remake has been at least 10 years in the works. Potter was unhappy with the way Pennies From Heaven was monkeyed around with for the Hollywood version, so he wrote the script for The Singing Detective himself, not long before he died in 1994. The original series, you may recall, starred Michael Gambon as Philip E Marlow, a bitter, misogynistic psoriasis-sufferer, covered in a lichen of painful flaky skin and sores, whose symptoms and sentiments are not unlike those of Potter himself. Drugged and immobilised in hospital, he fantasises about the hard-boiled detective novel he's written, scenes of which intermingle with childhood memories of his own unfaithful mother, and with reality. Characters suddenly slip into lip-syncs of 1940s popular songs. Way before Moulin Rouge and Chicago were touted as cinema's return to the musical, Potter was reinventing the genre with this and other television plays.

Potter's revised Singing Detective script gave the hero a new name, Daniel Dark; updated the flashback and musical period stuff to the 1950s; and substituted US-specific references for the Anglicisms. And, subsequently, got kicked from studio to studio, with a variety of big-name stars and directors attached. It finally landed at Mel Gibson's production outfit, Icon. Gibson decided to produce and co-star in the film as Dark's psychiatrist (which he did and does, in a laugh-inducing slap-head wig).

Having stayed on friendly terms with Downey ever since the two worked together on the Vietnam comedy, Air America, in 1990, he chose Downey to play the lead. How did Gibson's pitch to him go, I ask. "Mel came over, to the pad," Downey explains, "and in addition to making me some kind of green avocado bioflavonoid shitshake, he said [puts on Gibson accent], 'Yeah, man, I just got the rights to this. You want to have a look at it?'. I was like, alright, I need something to do this weekend. I asked for what, and he said, 'Maybe it's something we can do together. Anyway, I've got to split', and then he's taking off in a chopper."

Gibson paid the insurance costs for using Downey himself, which were set at an exorbitant rate because of his notorious history of going Awol on benders. The rest of the cast includes Robin Wright Penn as the protagonist's ex-wife, and Dawson's Creek's Katie Holmes in the Joanne Whalley role of the nurse, who gets ejaculated on by Downey's character while giving him a rub-down. The actor-director Keith Gordon directed the film, and insists that Downey was easy to work with. "Mel and Robert are so smooth and easy together, they're such good friends, it made those days flow really well," he says.

Downey is not exactly sure if he ever did see the original series when it came out. "It's not impossible that I did. When Mel gave it to me, I thought I'd heard of it or maybe even seen some of it." He watched a bit of it before he started shooting, but not too much lest Gambon's intense performance infect what he wanted to do with the part. Downey's version of the character is far more impish than Gambon's, even a little petulant, but then he looks much younger.

In the flesh, this 38-year-old shows signs of having lived it up a bit. In the film, he's on crackling and, literally, cracking form, what with the prosthetic make-up, grumbling and insulting all and sundry from his hospital bed like some kind of lizard king. At the end, he also shows off a rather good singing voice, which won't be that much of a surprise for those who've seen him croon on Ally McBeal, on which he used to be a regular guest (and, allegedly, the real-life lover of the show's star, Calista Flockhart) until his legal hassles got him fired. He claims that he writes music all the time, and has enough for a musical and a couple of albums, although he's shy about describing what his "sound" is.

Asked if it was a hard part, Downey goes off on a bit of a ramble [transcribed exactly here]: "It was, that it was such a literal piece of cinema and I didn't know how you... I mean, I think that Potter did a great job, but I think maybe that's a kind of generalised real British gift to do that, like Farenheit 451 over the thousand-yard stare thing, over the [John] Gielgud simmer. I didn't know how I was going to do that, to convey someone who's really angry and who cares at the end what the source of his disdain is. And doesn't it seem Freudian? Can we at least get up to Jung? You know, what the hell's going on here? Let alone postmodern, where they say [puts on prissy voice], 'Oh, you know, we get the play within the play...'. Can we get to [Joseph] Campbell? No, we're not ready for Campbell. Well, I think that every time. How? How am I going to do it? I think this every time. They're going to realise this is the thing I can't do. And then after that it's going to be, 'who gives a fuck anyway?'."

No, I'm not really sure what he's on about either, but I'm guessing that he's expressing some muted criticism about Potter's script (to which the film is largely faithful, bar the odd word), segueing into some kind of self-abnegation, leading to dismissal of said self-abnegation as indulgence. Anyway, the shoot was hard but over quickly on a comparatively trim, 35-day shooting schedule. Naturally, Downey had to spend up to four hours a day getting made-up to look suitably disgustingly scaly, which was itself painful. How painful? "By the time we got the tests out of the way, I was done tripping, 'cause I was, like, 'Let's get this on film because I feel like I have shards of glass Elmer-glued to my eyelids, you sadistic fucks!'," comes the reply. More of a shout really, so loud that the PR looks over from the other side of the room, worried that the star has been upset.

The pain must have helped him get into the role of the bilious, caustic Dan Dark, no? "Oh, dude, you could put me in a salt bath and the anger would be real," Downey says. "I'm practically always in a rage. But I try to be pleasant because I don't think its fair to be mean when you're working."

For most of the interview, Robin Wright Penn is fairly quiet, happy to let her co-star enjoy the spotlight. She seems to have come almost as a favour to him, to hold his hand and lend him support in front of the ratpack of journos. She does tell a hilarious story about her other co-star, Jeremy Northam's suggestions for how they should do their sex scenes together (he plays three different roles in the film). "He came up with all these ideas, he's so into the crazy stuff," she laughs. "So typically English, where they're so staid in life and then they come into work and they're like, 'I need an outlet!'. So he comes in and he's like, [English accent], 'Robin, Keith would you be too upset if I really need to twist my nipples – is that alright with you? Would that put you off? And then I'd really like to put my finger up your bum, would that put you off?'."

Downey and she explain that they weren't too respectful of the script, that they would transpose parts of their dialogue to the beginning of a scene, whatever felt right. "But I'm Marlon Brando on acid at this point. I've got cue-cards everywhere," remembers Downey. "Even on my forehead," Wright Penn pitches in.

Did his previous, ahem, experiences help him to get into the hallucinatory quality of the film? "Well, I'm definitely a big tripper. I love the 'shrooms," says Downey, frankly, and then seems to remember that he shouldn't be talking about that sort of thing too much. "When you're making a movie, you're not drawing on much, except how can I be as good as possible in this scene right now. I think that sensory recall is something you do for a couple of days, and it pays off in maybe one scene. For example, in one scene my character stares at the cracks in his ceiling, and there was a significant memory I drew on for that. My mom and dad once got into a fight and he threw a milkshake on the ceiling of our hotel.

"Even weirder on the double Oedipus reverb, when it dried it looked like a brown nipple. I remembered how uncomfortable it was to see my parents in serious conflict, where milkshake really wanted to be a pickaxe. That weird hate, hate, hate thing. So when we were doing the scene about the cracks in the ceiling, I just looked up and thought about that nipple and it fucked me up. That kind of drawing on the past thing worked. Apart from that, I have about four tricks, and half of them don't work anymore, so I probably need a new programme."

So, does this film feel like a comeback for Downey? "Don't call it a comeback!" he bellows, jokingly. "Tim Storey is a great inspirational speaker, beyond like a Christian guy, he does this thing called Hollywood Bible Study. Anyway, he says, 'the bigger the setback, the bigger the comeback'. I love that. But I don't know. I feel like nowadays, any time you complete one thing and move on to the next thing, the board is washed. You suck again, or you still suck. I think everything's a comeback.

"It's become incredibly and tenuous and precipitous out there, and you can't rest on your laurels. I remember one time I was in rehab and there was this gal there, and she had been at the LA women's correctional facility called Sybil Brand. She'd been mopping and she sat down to rest, and I asked, 'What you doing, baby?'. And she said, 'I'm just resting on my laurels'."

Soon it's time to wrap up – the photographer from AP is waiting to shoot him and Robin on the balcony. A quick, sincere-sounding, "Hey, this has been fun!", and a handshake, and he has already turned on his heel, lit a cigarette, and bounced out the door, moving so quickly that I worry the photographer's film won't be fast enough to catch him. He definitely doesn't rest on his laurels. Hell, the guy doesn't seem to ever rest at all.