Downey Jr: "I believed I was Superman"

Filming 'Iron Man 3', actor Robert Downey Jnr got a sudden attack of hubris. He thought he could do a wire jump without a rehearsal or a a harness. "We shot so many stunts I thought maybe I'm impervious. I'm 47 years old. What kind of moron says, 'I don't need safety harness, I'll just jump'? The next thing I know we have to shut the production down for six weeks. I really yanked my ankle. It hurt. Everything was a mess."

So he doesn't feel a superhero in real life? "Absolutely not. I mean I did, but not any more. I did that for five seconds and then I was in hospital… It's embarrassing. It was hubris. That's what it was. Oh God, not hubris again."

He may still have wild moments, but the actor, a Jewish Buddhist, insists he'll never battle addiction again.

In an interview with 'The Times',  Downey Jnr says his new-found emotional calm is due to his second marriage to film producer Susan Levin, while the endorphin high from intense martial arts workouts seems to have replaced the drug high on which he was once so dependent.

He compares it to being addicted to chocolate. "If you like chocolate cake and you know a really great bakery, you have to ask yourself if you are willing to pay the price of getting jacked up on sugar… If you've ever lost the ability to make that decision you realise that when you get that ability back it's the most precious and glorious thing you can possess."

His father, an avant-garde film director, introduced him to smoking marijuana as a child. "It was all great but the price was so high. It's a different generation, a different set of understandings."

Back in 1996, Downey was arrested for possession of heroin, cocaine and a .357 handgun after speeding down LA's Sunset Boulevard. There was a five-year run of arrests for drug possession and mayhem.

Eventually he served one year of three-year prison sentence. Even after he was released he was arrested for being on drugs when police found him roaming barefoot. He was written out of Ally McBeal. Mel Gibson cancelled a stage production of Hamlet in which Downey was to star.

Today he's back to earning $50 million a movie. "As I've gone on, I've become much less falsely confident. As you mature, things unravel. You start to address them. For me the process of maturing was very dirty, uncomfortable and embarrassing. But necessary."

He hopes to film anther 'Sherlock' with Guy Ritchie. " I love London. When we did the first 'Sherlock' we learned a lot about British people, which is if you're doing something difficult, don't just grind the monkey to the wheels come off. Be civilised. Offer everyone cheese. Always chat for a little while."

'Iron Man 3' is released on 26 April