For many years it looked as if one of Hollywood's most versatile and talented actors was on a self-propelled trajectory to his own destruction. It looks, with the briefest glance at his misdemeanours, that he's spent a lot more time either with his exasperated lawyers or explaining himself to a bemused judge somewhere, than he has making movies. But the man sitting before me smoking untipped cigarettes in Mayfair's Claridges, insists that he's made a clean break from his past and is now a changed man.

"I'm clean living now," he says, downing a huge latte, his fourth of the day. Looking assured and content after recently getting married to film producer Susan Levin, whom he met on the set of Gothika two years ago, Downey is busy throwing himself into a full working schedule, determined to put his considerable talents to good use again. He has just finished three films - Kiss Kiss, Bang Bang, Fur and A Scanner Darkly. And he's just about to start making Zodiac, which he describes as 'a gritty thriller'. He's even contemplating tackling the "greatest challenge of all", going back on the live stage again. "Yes, I want to do that, I truly do," he ponders, "if people trust me."

Of course, after Downey's long and well publicised fall from grace, it's convenient to forget that he had previously been Oscar-nominated for his work in Lord Attenborough's Chaplin, where he captured the perfect essence of the film legend. That was in 1992. And he was also described in one publication as "The most exciting actor of his generation", nowadays "the most unpredictable actor of his generation" would be more appropriate.

He's the first to admit that, a few years back, most Hollywood directors and producers would have combed through every list of performers at their disposal before they even thought of considering him for a role. But there are people out there - producer par excellence Joel Silver is one of them - who still have inordinate faith and trust in him. Silver later confides to me: "He is a comic genius. He understands the dark side very well. And so he should. He knows the extremes. He's been there."

Today, though, he'd rather talk of his life with Susan. He proposed to her on her birthday in November 2004 and they married in a ceremony on Long Island in New York. The honeymoon was a case of the newly-weds combining pleasure with business - they took themselves to the Deauville Film Festival in France where they busied themselves with promoting all of Robert's up-coming projects. His white gold ring gleams on his marriage finger, and when I ask what it means to him, it's the first time that you really see Downey slightly unsettled. "Err, everything," he says. "Everything. There's a Latin inscription inside and I am reliably told that it means 'til the wheels fall off the wagon'. I'm very happy with that."

But before the arrival of this newfound security (he describes himself as 'an actor who works to live - rather than lives to work'), he had a life that was the despair of his friends, family and fans. One of his worst moments was in 1987, when he was making the movie version of Brett Easton Ellis' novel Less Than Zero. "I was playing a junkie," he explains, pulling a face, "and the character was no less than an exaggeration of myself. Then things changed around and I became an exaggeration of the character. That all lasted far longer than it needed to." Then he says, totally unexpectedly: "Hey, time was, a few vodkas, a line of coke and a decent script, or tomorrow's working schedule, and I was anybody's."

It was Susan, though, who finally put her foot down. It was clear that the pair were in love, but "she only said she'd accept my proposal of marriage as long as I promised to stay sober and drugs-free," he says. "She is far smarter than me - and I like that. And love is a great therapy. Clinical therapy is good if you haven't got love, but you really do need a bit of everything, I think. Everyone hopefully gets the formula they need. Or am I just lucky?"

He certainly wasn't lucky with his first marriage, which floundered because of his many dependencies. His first wife was actress Deborah Falconer, and they have a son, Indio, who is now 11 years old and "obsessed by Manchester United." He readily concedes that he isn't used to being happy and finds that contentment is a bit of a new experience for him because "I was so used to getting stoned for the past 14 years. I should probably be doing meditation constantly for the next four years, with a few breaks every now and then for food . Susan understands me and calls me 'a work in progress'. She knows that it will be some time before we will both be entirely happy. She can cope with that, she says. I am a person who has had no self-esteem, and now I am in an area of low-esteem, but I am on my way."

Susan is certainly a very determined and feisty lady. And she keeps a vigilant eye on her new husband. They share a small apartment in New York where her scripts and his mingle on the massage table that they have in the centre of the living room. "I am a very domestic animal these days," he smiles. "I take a delight in seeing how the house is ordered. I am not a n'eer do well anymore. If I leave the house with my hair washed and my shoes on the right feet, then these days I am in good shape. The next thing I want to tackle is flower arranging. Or maybe embroidery. Oh, God, how I want to see that in a headline somewhere."

Perhaps his fans and most people in his industry recognise that, miraculously, the only person he's been really hurting over the past few years is himself? "Yes," he agrees, "and I like the word 'miraculously'. I've found - somewhat later than I should have done perhaps - that humility is the key to success. I thought back at the time when I worked with Dickie Attenborough and I realised what a great mentor he had been. I only wish I could sit at his feet for two or three weeks again. What a great man. We should listen to our elders and our betters a lot more."

Perhaps a sign of this new-found humility is that he's quick to add that he wouldn't recommend his past way of life to anyone. "No way," he says. Not unless they were very daring and very ignorant. I think that I've been through a lot, that I've been tested, and that I've paid my dues. And like the Sondhiem song in Follies, 'I'm still here'" His keen sense of the ironic has also helped him. In Gothika he played - of all things - a psychiatrist. In A Scanner Darkly he plays one of the most balanced and sane characters. And in Fur he is an exhibit in a circus sideshow, constantly being stared at by the public. Art holding a mirror ta life - or what?

Not surprisingly though, Downey's still smiling. "Oh yeah, definitely," he says. "Not a cocky smile, I hope. A relaxed one, yeah." He gets up, shakes hands and leaves. The door closes. And then it opens again. "Don't forget what I said about flower arranging and embroidery," he grins. "I mean it. Open up one of the celebrity glossies in a couple of month's time, and you'll find me there." The door closes again and in the corridor you can hear Downey lost in a huge cackle of laughter.