Based on a graphic novel written by Alan Moore, V is set in a contemporary Britain ruled by a fascist government. A lone, masked revolutionary, V (played by Hugo Weaving) is sworn to bring down the government by any means. When he rescues young Evey (Portman) from the clutches of the secret police, she begins a long journey into the soul of the revolutionary. Finally, she accepts his ideas, shaves her head and prepares to do battle with the state.

'People respond in very different ways to this film,' says Portman. 'To some people, it relates to the current political situation. But I've met people who think it's about Hitler's Germany. A South Korean reporter I met was convinced that we'd made a film about North Korea. Everybody comes out of the cinema with their own opinion.'

Portman says her Jewish heritage gave her a lot to think about during filming. 'Someone once said to me that one of the most Jewish things is to try to understand why someone else hates you,' she says. 'But Jews have been called terrorists, too - like when they fought against the British occupation of Palestine. I think we can all imagine a situation where violence is legitimately used to express political outrage. It's perhaps where we set the threshold of what we do that matters.'

That the film was shot in Germany - in Potsdam's huge Babelsberg studios - was also food for thought, she says. 'I loved working in Germany, but I was nervous to go there,' Portman says. 'After all, the Holocaust is part of family lore. I'd never spent any time in Germany before, but I liked Berlin. It seems very aware of its Nazi past, but it's also ready to create a new story for itself. It was interesting to shoot this film in a city that had actually had a real totalitarian government.'

The film features bombs on a subway train, but this was already in the script before Islamic terrorists exploded bombs in London's Underground.

'Obviously, every time there's an act of violence, it's upsetting,' says Portman. 'It makes you think about how you depict violence on the screen. You have to think about the relationship between the screen and violence. But it didn't change the film at all. A lot of the events in the film are taken from the novel, which was written in the 1980s. These things seem to reoccur, so it makes no sense to shy away from them.'

Portman says the film's head-shaving scene was stressful. 'It had to be done in one take, obviously - we couldn't go back and reshoot,' she says.

Portman says she still has fond memories of The Professional, the film that launched her career. 'I would have done a toothpaste commercial if it had come up - I just wanted to be an actress. But I got to make a film with Luc Besson and Jean Reno instead. It's a great movie.'

The Professional set a standard for the sorts of films she wanted to make. 'It was so good,' she says. 'It gave me something to aim for. I rarely watch films after I've made them. But I like watching The Professional because I'm just a kid in it. I look like a different person. It's fun to watch yourself on screen when you're a little kid. That's not something many people get a chance to do.'

Her fans seem to agree about The Professional. 'I think that the most surprising thing is that, even though I was in the Star Wars prequels - the most-seen movies on the planet - people still come up to me to talk about The Professional,' she says. 'That's still the one that people are interested in. But it's always confusing what films people pick up on. A lot of younger kids recognise me from Garden State. People who watch TV a lot recognise me from Where the Heart is, which gets a lot of TV exposure.'

Portman typically plays smart girls, and that's the way she likes it. She says she's concerned that fewer independent-minded characters are being written for women. 'I'm not interested in playing female characters which make women look silly,' she says. 'There's no lack of silly girls on screen, unfortunately. I try to avoid negative types of women, or cliched stereotypes like the brainy but sexually repressed woman.'

Nonetheless, she says she likes what she calls cheese-ball movies such as Dirty Dancing. 'I grew up with films like that,' she says, looking a little bashful. But she doesn't consider the genre or style of a movie when picking a script. Her next film will be Goya's Ghost, directed by Milos Forman. Then she'll make a children's movie.

'I just like good movies, and I don't draw a distinction between big films and small films,' Portman says. 'You never know how a film will turn out when you're making it. I choose films that will give me different experiences, and I try to pick films that challenge me. If it's interesting for me to do, then I think it will be interesting for an audience.'

Portman disagrees with the idea that V For Vendetta is a negative prophecy about America's future. She says she likes the US and has faith in the future. 'I'm more optimistic than to think that today's America is like the one portrayed in the film,' she says. 'We can find resonances of V in today's world, but I think that America will turn out a little different to that.'

V is for Vendetta opens on Thursday