Kevin Spacey explores his dark side once more in 21, this time as a maths professor who leads his students on a card-counting cheat scam in Las Vegas. But for this dynamic character actor, it's just another typical gamble.

While Kevin Spacey may have moved to London with a ten-year plan for revitalising the fortunes of the capital's illustrious theatre, The Old Vic, the actor and occasional director's film career has been in need of a little resurrection itself. Since his Oscar winning performance in American Beauty, Spacey's appearances in lacklustre fare like K-Pax, Pay It Forward and The Life Of David Gale have been instantly forgettable. Now, after seducing audiences with his memorable portrayals of sociopathic villains in Se7en and The Usual Suspects, and making mid-life crisis cool in the aforementioned American Beauty, Spacey is again looking to invest pathos in characters who tap into a darker sense of morality. If his recent portrayal of Lex Luthor in Superman Returns was his grandiose villain, then Spacey's latest role in 21 proves that even unassuming mathematics professors can have a dark side.

You're also a producer on 21, and have been planning to make a film on the true story of the students who took Vegas for over a decade? What took you so long?
"Yes, my business partner [Dana Brunetti] and I had been talking about it for twelve years. We started hearing these rumours about this group of kids who would go to Vegas, but we could never find any evidence. My producing partner wrote a treatment twelve years ago, but it never went anywhere. Then one day in 2000, he was walking past a newsstand and there was Wire Magazine with the cover line, "The true story of the Massachusetts Institute Of Technology (M.I.T) students who took Vegas for millions." And we were like, 'We didn't know that the writer was Ben Mezrich, or that he'd written a book [Bringing Down the House: The Inside Story Of Six M.I.T Students Who Took Vegas for Millions] that ultimately came out two months later.' So Dana Googled him and got a number in Boston. Ben thought it was a prank call, but after he Googled Dana, he called back, and then he was on a plane to Los Angeles."

So what made you want to act in the film as well?
"I liked the whole idea of this stuffy M.I.T maths professor who's actually got a whole secret life going on. He's not based on a real person, but he's an amalgamation of a bunch of people, so there was a chance to have fun with it. It was insane what they could get away with. This obviously all happened before there were issues with terrorism, but these kids would literally put hundreds of thousands of dollars all over their bodies and go through airport security on the way from Boston to Los Angeles."

Is it difficult to convey the actual thrill of gambling to an audience?
"There have been a lot of poker movies, but poker is too hard for me on film, as the game's too difficult. It's easier to make the game '21' understandable for the audience, by trying to visualise what the game is. I know how the game really works, because we went on research trips to Vegas with the actual guys in the original book."

Trying to double The Old Vic's money?
"Exactly! The casinos wouldn't let the guys play, but they didn't mind if they were standing behind us, so I had them stand very close to me, and when they wanted me to up my bid, they just pushed my chair. And every single time, I won! Every time! I want them to travel with me every time I go to Las Vegas. I'm not good at maths, but their system is astounding."

You performed the David Mamet play Speed The Plow recently at The Old Vic, which centres on the issue of art versus commerce. Considering the struggles of financing The Old Vic, do you sometimes think of doing film roles to boost your profile in order to help the theatre's cause?
"Well, not that much has changed in terms of my roles. Some of the movies I'm doing are because I've never had that experience before. I did Superman Returns because (a) I got to work with Bryan Singer [The Usual Suspects] again, and (b) I'd never done a big tentpole movie. I wanted to see what it was like to have the experience where they're spending money like you can't believe. It was insane: there were sixteen sound stages and we were on all of them. I'm certainly not making any money doing what I'm doing at The Vic, so, yeah, being paid does come into it, but it's not the first consideration."

21 is released on May 15.