'I thought he was nuts," says Thandie Newton. She's talking about Oliver Stone's surprise decision to cast her as Condoleezza Rice in W, the George Bush biopic that's set to burst into cinemas in October, a couple of weeks before America goes to the polls to elect a new president. "I hardly knew who Rice was, apart from being secretary of state."

It's rather like Republican candidate John McCain deciding that self-proclaimed hockey mom Sarah Palin could learn everything there is to know about international affairs in a couple of weeks. Of course, all Newton needed was enough to give a performance.

"I became completely absorbed," she says. "I read everything I could find. It was like being back at Cambridge researching a paper. I read all the articles and books I could find. I plugged into Wikipedia and YouTube. I could have gone on Mastermind on the subject of Condoleezza Rice and the Bush administration from 2000 to 2004."

Stone took much longer convincing Newton she could be a believable secretary of state than McCain did getting Palin to be his running mate.

"I was terrified, genuinely terrified," she says. "Rice is decades older than me. She doesn't look like me. And I'm not even American, I'm English. But I loved the responsibility and the chance to be a part of history. I knew I could do it."

Stone chose Newton rather than a mixed-race American actress partly because she didn't have an instantly recognisable persona that might detract from audiences seeing her as Rice. She possesses the great ability of a character actor to lose herself in whatever role she plays so that it's never about her.

"I like having to create someone from scratch rather than just looking like someone. It gives you more scope. I love actors like Gary Oldman who can transform themselves. Your head is spinning. You see them in so many genres. Who are they really?"

Newton, who first caught Hollywood's attention in Toni Morrison's Beloved, has starred in an impressive range of roles whether as a pampered LA wife sexually molested by a racist cop in the Academy Award-winning Crash, or Tom Cruise's sexy spy girl in Mission: Impossible 2, or a slave girl in Jefferson in Paris. The daughter of a Zimbabwean healthcare worker and an English laboratory technician, she was educated at Downing College, Cambridge, but made her screen debut at 19 as an Australian schoolgirl in John Duigan's Flirting, which also launched Nicole Kidman. Up to then, having grown up in Cornwall where her grandfather had an antiques business, she had never encountered racism.

"There was a scene where all the kids in the dance had to move away from my character. I remember asking what the logic of the scene was and being told it was because I was black."

Nobody seems quite sure who Thandie Newton is. She can look Asian or African, but in conversation is thoroughly English. Most of her screen roles are American.

"As an actor, it helps not really belonging anywhere," she says. "An actor friend of mine once joked, 'you know what, I wish I was an ethnic minority like you'."

Now 35, she's married to Ol Parker, an English director she met when she featured in his 2000 debut film It Was An Accident. They have two daughters, Ripley, aged seven, named after the Sigourney Weaver character in Alien, and three-year-old Nico.

"I wish I could get more work in England. Ripley really loves going to school here. But it's very hard to make a film here of a size that's going to compete with American films. We just don't have the audiences. If you're an English actress you serve your apprenticeship on TV and as soon as you can try and get an agent in America and work there."

Guy Ritchie's gangster romp RocknRolla, produced by Hollywood veteran Joel Silver, is one of her rare English screen appearances. She plays a svelte but crooked accountant who works for a Roman Abramovich-like Russian billionaire.

"I'm nothing like her," she laughs. "She's very much a character drawn for entertainment. She's all about control. She has lots of money, a great job and she gets to sleep with whoever she wants. I'm probably more about surrendering to the way things are. I don't want to detach myself and escape into any other place."

"Thandie is posh, isn't she?" Ritchie told me. Perhaps that's what Stone sees in her too, a sense of the gravitas you'd expect in a secretary of state. The name Thandie means 'beloved' in most African languages. Her mother reputedly comes from a line of Shona princesses. Yet confidence has only come to her with experience.

"I'm better now at juggling the two sides of being in the public eye and being at home. I've sort of merged with it the older I've got. I'm more comfortable in a situation like this, giving you an interview. I used to think I was on trial all the time, having to say the right thing."

Stone, who was at Harvard at the same time as Bush, began shooting W only in May, but has it ready for release in the States on 17 October. He was about to film Pinkville, an investigation of the My Lai massacre in the Vietnam War, but United Artists shut down the production because of the Hollywood writers' strike. So he seized the chance to rush through the Bush project with non-American financial backing. He has likened W to the behind-the-scenes concept of his 1995 film Nixon, but closer in manner to The Queen.

"I want a fair, true portrait of the man," he told Variety. "How did Bush go from an alcoholic bum to the most powerful figure in the world?" Elsewhere he has promised to strike a satirical tone similar to Sidney Lumet's Network or Stanley Kubrick's Dr Strangelove. So what can we expect?

"It's hard to say. Don't ask me about that," laughs Newton. "I'm not really able to comment because the editing is going to have so much influence over the final tone. Oliver shoots a lot. History was happening all the time. He would add, add, add. I don't know how many of these inserts and extras will go in. It was hypnotically tense on the set during filming. Most of my scenes are with Josh Brolin, who plays Bush, and it was about that relationship between Condoleezza and Bush and that loyalty between them. I don't know how it's going to turn out."

Has she ever met Rice? "No." Did she develop sympathy for her? "No. That's not to say I feel critical, but I don't feel sympathy. They have a job to do, to do right for all the people they're governing. That's how they will be judged."

She's excited that W is coming out right before the election. There are plans to run TV spots alongside John McCain's ads. She's not daunted by the prospect of being caught up in all the controversy. "History is happening right now. America is about to go through another completely new phase of government. It's intoxicating to be part of that debate."

'W' opens in Ireland
on 8 November. 'RocknRolla' is currently on release