August 13 2011: Mamma Mia! star Dominic Cooper tells Amy Raphael about flatshares, fame and sex scenes with Keira Knightley

Dominic Cooper turns up with a Mark Ronson quiff and a body so buff that, when he twists his torso this way and that as we talk, his shirt gapes a little to reveal a perfect six-pack. It’s no wonder, then, that the actor whose role in Mamma Mia! made him a global star doesn’t mind going naked on screen. “Nudity can of course be terribly distracting, but it’s never a problem if it’s necessary and I feel comfortable with it.”

I tell Cooper that an actor once told me how he was offered a choice of three special socks with which to cover himself during a Hollywood film heavy with sex scenes. He laughs. “You can often be Sellotaping a string between your buttocks to hold your … thing between your legs. It’s madness. I’d never go completely nude because it’s inappropriate for the actress you’re working with. The actresses I’ve worked with have been wonderful; they’ve all said, ‘Whatever happens happens. Don’t worry about it’.”

So in the sex scene with Keira Knightley in The Duchess, did he manage to stay in control? “Yes … no. I don’t know! I’ve never had an erection on set. I know people who have. Maybe the lack of actual physical contact helps:the actress wears something to cover herself too. At least you can both laugh about this ridiculous situation you find yourselves in, your bits covered by contraptions that often fall off. You have to get on with it and ultimately trust the director.”

And accept it as part of the job? “Yes, of course. But at the same time being nude in a film is extraordinarily awkward and weird.”

Cooper, 33, takes acting terribly seriously. Since leaving Lamda (the London Academy of Music and Dramatic Art) in his mid-twenties and working pretty much non-stop he has proved his versatility. He impressed on stage first, in The History Boys by Alan Bennett, in 2004, and then opposite Helen Mirren in Phèdre, in 2009. He can be funny — most memorably as a Goth drummer in Tamara Drewe — and, more simply, a pin-up. Playing Sky in Mamma Mia! may not exactly have exploited his acting skills, but it did catapult him into the spotlight. It’s the role that continues to define him.

Cooper’s latest role is about as far from pop songs and Greek islands as it is possible to imagine. In The Devil’s Double he gets to be in nearly every scene by playing both Saddam Hussein’s eldest son, Uday, and his body double, Latif Yahia. Yahia is an unwilling accomplice forced to abandon his family and his simple life. He has minor plastic surgery to make him look like Uday so that he can appear in in place of the dictator’s out-of-control son. It’s a disturbing but glossy look at Iraq’s tragic recent past. “Lee Tamahori, the director, was very clear that The Devil’s Double was never going to be a biopic,” explains Cooper. “The film was never supposed to be anything more than a gangster film.”

Was it a challenge not to reduce Uday to a caricature? Given that he has goofy teeth and a camp manner, was there a danger of it becoming a bit like Sacha Baron Cohen doing a Freddie Mercury impression? “I had to go that far. Everything I learnt about Uday showed he was a joke. In fact, we had to calm down some of his more ostentatious behaviour. He truly was a hideous individual.”

When Cooper got back from shooting The Devil’s Double he did what he always does when a job ends: he read some books, went to the theatre and played squash, tennis and football. And he checked in with his mum, who lives in Greenwich in southeast London, where Cooper was brought up. His parents separated when he was young; he has two older brothers and a half-sister, the latter a relatively recent discovery after a drunken revelation from his father.

Cooper sounds slightly in awe of his mother, Julie, who spends much of her time travelling alone. “I never see her quite enough. She’s in France now. I’m hoping to get down there to see her next week.” And what of his father? Are they close? “Everyone gets on; my extended family came to the premiere of The Devil’s Double. My dad still lives close to my mum. But I’m very similar to my mum. She has strong opinions and she’s not afraid to voice them. I’m the same. So we do spark off each other. It can be tricky. But we laugh a lot, too.”

Cooper lived at home with his mother until he graduated from Lamda in his mid-twenties. He then rented a flat in North London with James Corden, who became a friend when the pair appeared together in The History Boys. Cooper and Corden decided that they loved their street so much that they each bought a property on it.

But Cooper left his own place empty and instead moved in with Corden. He didn’t even leave when Corden got engaged and was about to become a father. “I don’t know what I was thinking! I was away an awful lot, but” — and here he laughs at himself — “I’d come back and find more and more baby toys in my bedroom. I suppose James and I had lived in chaos but it had been such good fun. I was reluctant to let that chapter in my life end.”

But the bromance didn’t end, it simply became less intense. Cooper has even left a box of toy cars and school books in Corden’s house. He mumbles about heavy boxes and too many stairs, but it’s almost as if he can’t quite bear to extricate himself. Yet he insists that he loves living alone. “I loved it as soon as I moved in! I’m extraordinarily happy. And James is just down the road.”

Corden now has a four-month-old son, and it would be easy for Cooper to be envious of Corden’s new family life. Instead he says that he’s relieved for his friend because Corden always wanted a family and, anyway, it was Cooper who introduced Corden to his fiancée, the charity worker Julia Carey at a party. Ask Cooper about starting a family, though, and he jumps right in: “No, not yet.”

He quickly adds: “I’m really looking forward to it though. Actually, I can’t wait. I love kids, I adore my nieces and nephews. It will be a really exciting chunk of my life. I know that, ultimately, it will give me a great sense of purpose in life.”

This is all he will say about his private life. He won’t say whether he has a girlfriend. He talks at length about wanting to keep his personal life private, of being baffled by media interest in his relationships. I think his reluctance is in part caused by an almost naive (and quite charming) openness in early interviews, when he talked frankly about Joanna Carolan, his girlfriend of 12 yearsand a former PA to the late Harold Pinter. The couple split after he fell for his Mamma Mia! co-star Amanda Seyfried. When Mamma Mia! came out their subsequent on-off relationship led him into the tabloids, a place he has no interest in being.

Although he is almost offhand about the experience of having his every move with Seyfried analysed in the press — “it didn’t burn me. It didn’t shock me. It didn’t disturb me. I just avoided reading about it” — it does mean that he will not be drawn on dating. Instead we chat about the false nose he wore in The Devil’s Double (“It was a better nose; it’s the operation I should have to sort out my ski-slope nose”) and the recent unrest (“it’s hard not to see those kids as a bunch of savages”).

Then I tease him about a story I read about Corden’s new best friend, David Beckham, being godfather to his son or best man at his wedding. Cooper looks genuinely shocked. “Becks? I can understand him being godfather, but … best man?”

Would that be the end of your bromance? He pushes his hand through his black hair, making the quiff even bigger. “How is anyone going to write James’s best-man speech? How is it ever going to be funny enough? He’d have to write it for the best man himself. It’s the only way.”

Before Cooper has his photo taken, I ask about his plans. “I have no idea what my next acting job will be, but I’m not worried.” He looks utterly relaxed, the antithesis of the neurotic actor. And then he smiles. “I’m just looking forward to hanging out with my mum.”

The Devil’s Double is out now

Dominic Cooper’s perfect weekend

Takeaway or home-cooked meal?

Home-cooked meal. I don’t like following recipes. I like being creative with ingredients. When I see the damage I’ve done in the kitchen, though, I probably wish I’d had a takeaway.

Waitrose or Lidl?

I’m not organised enough to do big supermarket shops. So, local shops, local produce.

Sun or ski?

Recent skiing holidays involved me going down a mountain on my bum trying to look cool while snowboarding. Then I went back to skiing and realised how wonderful it was. But, given a choice, I would always go for sun.

Night in or night out?

I love both in equal measure.

E-mail or snail mail?

What’s snail mail? Oh, post. Fax machine.

Yoga mat or football pitch?

Football pitch.

London or LA?

London.

Board game or video game?

Board game.

Cornwall or Caribbean?

Caribbean.

What are you reading?

A World on Fire: Britain’s Crucial Role in the American Civil War, by Amanda Foreman.

I can’t get through the weekend without . . .

I know what I’d like to say. Instead I’ll just say laughing.