At 43, Robert Downey Jr is a far cry from the drug-fuelled days of his youth. Laid back in a leather armchair at the Four Seasons Hotel in Beverley Hills, he is looking relaxed and at ease. Pointing to his army-style outfit he explains:

"I pulled this jacket out of the closet this morning. They let me keep one jacket, but they didn't let me keep the wig. I think that's actually going to be in a museum. My museum."

In his latest movie, Tropic Thunder, Downey Jr. plays respected, Oscar-winning Aussie actor Kirk Lazarus. Dreamt up by Ben Stiller - who co-stars, directs, co-writes, co-produces and probably acts as animal wrangler as well - Tropic Thunder imagines a lost patrol of self-absorbed yet terminally insecure actors working on a war movie to end all war movies in Southeast Asia only to wind up in a real-life combat with narcoterrorists. The end result is a glorious send up of all things Hollywood. But, despite the distinct likeness to Crowe, Downey swears his character was not based on his volatile peer Crowe.

"I certainly wasn't trying to impersonate him and I have such high esteem for the guy. I honestly would feel so bad if he was in any way offended because it wasn't my intention."

Lazarus goes through a skin pigmentation process to fit into the African-American role of Osiris, the character he plays in the Vietnam war epic film within the film, that also goes by the title of Tropic Thunder. Downey Jr. had obvious reservations about changing skin colour for the role.

"I think it's entertaining and I think it was thoughtfully done. Sure, I was hesitant when Ben asked me to play the role. If you see the movie, it's a non-issue. But if you'd read the script first, you would say, 'This is probably a non-issue, but there's a lot of things that have to be managed and handled correctly, down to the look and the execution.' There were times I was a little freaked out, like if I do this wrong, it's kind of my fault. Given the fact that Ben Stiller directed it and all these other weirdos were around him, it's practically impossible that it turned out as well as it did."

And it wasn't just his appearance he changed for the role.

"I had to change my accent a number of times. While I was Kirk, I had an Australian accent - but as Kirk's character Osiris I had to have a strong African-American accent. There was a lot of to-ing and fro-ing. I loved doing the Aussie accent for the part of Kirk. For some reason, I found it really easy. I admit didn't find Osiris' mumbling voice as easy, but it just happened. And once it did, it was this frequency that the entire character came down. Honestly, I didn't know what I was doing one day and then suddenly I called up Ben and started talking and he's like, 'That's good.' And I could tell by the tone in his voice that he wasn't placating me. Then it just happened and once that voice came through, everything else was really easy, because I just felt like the guy was saying what he wanted to say. It also took the pressure off me because I was very ornery when we were shooting and if I was Kirk Lazarus, I was kind of looking around at people thinking [adopts character's Australian accent], 'I wonder if there's anyone else half as talented as me here…' And when I was Osiris, I was like [adopts heavy, mumbling African- American accent], 'Dis is f***in' bullshit…' So I got to have a lot of fun."

For years it looked as if one of Hollywood's most versatile and talented actors was on a selfpropelled trajectory to destruction. But now it seems Downey Jr. is a changed man. He has a full work schedule ahead of him, he proudly says that he is now "clean living" (except for a cigarette or two, and lots of strong espressos) and he is blissfully married to film producer Susan Levin, whom he met on the set of Gothika five years ago and whom he credits with saving him from his worst enemy - himself. Downey Jr is back where he should be - at the top of his profession and in demand. But is he enjoying being back on top in Hollywood?

"Yeah, I'm so on top of it, I'm in the missionary position," he says in a sexy voice. I've been pretty much in every angle corresponding to Hollywood and it's just great. My experience is that actresses, actors and directors tend to either get better or worse over time and if I've just gotten to the top of my game 25 years in, then it would probably indicate that I'm not getting worse. I just have to stay dialled in and everything will be cool."

It is no accident that Downey Jr. has such a huge talent for the big screen. His father is an actor, writer, producer, cinematographer, and director of underground films, while his mother, Elsie, is an actress. Because of his connections, he made his first appearance on the silver screen at an early age in minor roles in his father's films, before taking classes at a New York acting academy and making his way along his rocky road to fame, fortune and, in his case, addiction. Now he says he has been drug-free since 2001 thanks to the help of his family, therapy, meditation, 12-step recovery programmes, yoga and the practice of martial art Wing Chun Kung Fu. And his clean living seems to be working, not just in his personal life, but professionally too. Since landing the role of Tony Stark in Iron Man, Downey Jr is in such demand that he's commanding $12.5m a movie, and has been working back-to-back on different projects.

"Seriously though, it's great being busy. It's a lot of work and it's anxietyprovoking, I need to be very focused and disciplined. I think if you don't manage your anxiety, everyone can tell because you act out and those tend to be demonstrated as incidents. Public or private. So, managing anxiety is about expressing anxiety rather than trying to contain anxiety. Anxiety to me means underneath it all there's something I'm scared of, and that adds up to irritation and confusion. I'm scared I'm going to lose something I already have or I'm scared that I'm not going to get something that I want. My thinking process is so twisted by nature, so I always have to keep riding it because I'll wind up so far off course. I have to be careful what I wish for. Here I am, promoting a movie, while my heart says I should not be promoting Tropic Thunder at all because I need to have my ham radio going ready for my next role. But you can't say that to a studio: 'Listen, I have a ham radio inside my psyche and I'm looking for inspiration.' It might happen in the three days that I'm doing this press and what if I miss that train?"

But Downey Jr. is looking forward to the future - especially beefing up for his return as Marvel hero Tony Stark in Iron Man 2.

"To reprise Tony Stark is great. Actually, I'll be fortunate if I can just manage to do everything that I have lined up to do correctly."

The first role Downey Jr has accepted since the Iron Man is the lead in Guy Ritchie's Sherlock Holmes.

"I can't wait," he says. "I love the idea of doing a period piece without trying to be too stylised. The accent hasn't come to me yet. Because I've done classic English accents before, in Chaplin and Restoration and all these movies. But I don't want to just do that. I feel Sherlock Holmes, whatever that frequency is, is out there somewhere and I got my little accent radar on and I've got nothing yet. It's kind of a process of elimination. We start shooting at the start of October, so I should have it by then."

Downey's face lights up as he explains he is being given the chance to combine acting with one of his other passions in life, martial arts.

"Guy and I are both martial arts enthusiasts and historically, in the real original stories of Sherlock Holmes, Holmes is a kind of a bad-ass and a bare-knuckle boxer and studies the rare art of baritsu. If you look it up, they can't even really tell you what it is, so it gives us a lot of leeway."

But this Hollywood superstar is looking forward to developing his brains as well as his brawn in his maturing years.

"Besides making films, I do love music. But I'm so uneducated in things like, you know, just general culture. Even when we were in Paris and we got to go to the museum where all those Monets are - I don't even know anything, so when I go out and see like his water lilies painting, I go, 'Oh, that's what people have been referencing when I've been at a dinner party....' That's the downside of the whole movie thing: it's not a real life as far as absorbing anything besides the culture of making movies. I know some people are really refined and educated, but I'm not. So that's a passion of mine and will be for years to come. I'm actually kind of glad 'cos I'm almost in my mid-40s and that's the type of stuff that I can enjoy more now than if I was travelling out of college. I just want to see the world and see what everyone's been doing here the whole time."