Robert Downey Jr may have left his bad-boy days behind him but it would be hard to say he’d mellowed. He speaks frenetically, and at one point starts shouting, leaping around, swearing and jabbing wildly at his BlackBerry as he unsuccessfully tries to stop it beeping.

A reformed drug addict who has done time in prison, Downey, 43, has now been cast in the unlikely role of a superhero – the hardest thing he’s ever done. He had to get his body back into shape for the role after years of abuse. He started young, when he was handed a joint by his father at the age of eight, and eventually spiralled into heavy drinking and cocaine and heroin abuse.

For the film Iron Man, based on a character from Marvel Comics, Downey endured intensive workouts – “probably 1,500 hours of effort for 11 seconds of screen time”, he says – to prepare for the role of the billionaire playboy Tony Stark, who becomes the armour-clad avenger Iron Man.

“I did tons of weightlifting,” he says. “I do a lot of martial arts anyway, so I stepped up the old Wing Chun a bit. It’s always nice to say I’m doing it for a movie. It’s always great when you get to keep fit at someone else’s expense.

“I also quit smoking for the role, although once it was done I started again. Now that I’m promoting it, I’ve quit again.

“I think it’s probably better not to smoke?” he questions, as if not entirely convinced, though he says categorically that he is now drug-free.

Some might consider Downey to be past his prime in comparison with recent superheroes such as Tobey Maguire’s pubescent Spider-Man and Brandon Routh’s fresh-faced Superman. But he has nothing in terms of age on Sylvester Stallone, 61, or Bruce Willis, 53, both of whom can still claim action-hero status. Downey refuses to accept that he’s past it: “Your hubcaps tend to fly off more when you’re taking corners in your forties,” he says, presumably meaning that his body doesn’t absorb the shocks the way it used to. “But somehow or other I’m in pretty good shape.”

He enjoyed powering around the set of Iron Man in an Audi R8 and claims he could be in line for one – a step-up from his Mercedes-Benz E-class – as soon as they go on sale. “Although my Mercedes has a huge engine and is as big as a European taxi cab,” he says appreciatively.

Referring to his huge, sweaty superhero suit, Downey says incredulously: “There was just the one suit for me and all these other stunt guys. One guy was tall and had a smaller head, so the helmet was big on him, whereas it fitted me just right. And then another guy had bigger feet and a bigger head, so he had a headache.

“Most of the time I was standing on my toes because of the tall guy.”

That was nothing compared with the hassle of getting into it in the first place. “The suit went on piece by piece. First there’s this thing that’s moulded to your body. Then two pieces that fit together on the back and front of your chest. And then they hang the shoulder pieces off that. Then you put your arms on, and they powder your fingers and slide you into the hand pieces. So now you’re just loving life . . . ” He laughs. “Then you put the hips on, followed by the legs and calves. Then the boots go on, and finally they screw all that together.

“Then the helmet goes on, which is kind of like trying to put this over your head,” he says, lifting his teacup. “And then it closes, and you’re good to go. By now you’re tripping because you realise, ‘Shit, it’s going to be so hard to get out of this thing’.”

I suggest they will perhaps fix the suit for the sequel, and he replies, deadpan: “You would think so, wouldn’t you? Depends on how much money it makes . . . ”

The son of the underground film-maker Robert Downey Sr, Downey Jr made his film debut at the age of five as a puppy in his father’s film Pound. Mainstream early successes came with Weird Science and Less Than Zero, before he achieved critical acclaim with Chaplin, earning an Oscar nomination and a Bafta award in 1992.

In 1996 he was stopped for speeding, then arrested for drink-driving and possession of heroin, cocaine and an unloaded pistol, eventually spending several months in jail from 1999 to 2000 in the wake of a number of probation violations. Even at the height of his late 1990s drug spiral, which culminated in several months in jail, he managed to pull off convincing performances in The Gingerbread Man, Bowfinger and Wonder Boys. Then, post rehab, he cemented his comeback with Kiss Kiss Bang Bang, Good Night, and Good Luck and Zodiac.

Having dated Sarah Jessica Parker for several years in the 1980s, Downey married the actress Deborah Falconer in 1992, and they have a 14-year-old son named Indio. They divorced in 2004, and the following year he walked down the aisle again, this time with Susan Levin, a film producer he met on the set of Gothika.

He’s still thrilled to be given a second stab at Hollywood: “I remember, when I first got cast [in Iron Man], I was so excited that Susan – Mrs Downey – was, like, ‘You’ve got to sleep. You’re like a baby that won’t go down for its nap’.”

In his new £75m blockbuster Downey heads up an all-star cast that includes Gwyneth Paltrow, Jeff Bridges and Samuel L Jackson. Just eight years ago, amid drug charges and jail, it seemed unlikely that he would ever be entrusted with such a valuable franchise. But Hollywood can be quick to forgive. “Don’t they say, the bigger the setback, the bigger the comeback?” he muses.

“But I wouldn’t necessarily recommend that people take that to heart. Like Tony [Stark], I took some hits, mostly of my own making, but everyone transforms; some of it is just a function of age. I’m in great shape, I have tons of energy and more gratitude than I can even express.”

My stuff...

On my CD player
David Gray, Elvis Costello, Dave Matthews, Yes, Elton John and my own CD, The Futurist

On my DVD player
I’m a movie nerd. I love them all and watch everything

In my parking space
A silver Mercedes E-class (right) with a 6.3 litre engine. In Iron Man I drive the new Audi R8 – I think they said they’d give me one when it goes on sale but we’ll see. Whatever

I will never throw away
My BlackBerry. Ha! I’m joking, of course