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. 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..... "

He's 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.