Helena Bonham Carter, seated in the restaurant of the Covent Garden Hotel in London, easily holds a stranger's gaze across a starched linen tablecloth. Contrary to the impression left by a string of doe-eyed ingenues she's played on screen, Bonham Carter (the hyphen, she insists, is optional) is not given to blushing or pushing garden peas around her plate. "People have misconceptions about me" she says. "Lots of them. My first live interview was to promote Room With A View on The Today Show. The interviewer went 'Tell me what it's like to be royal?'. 'Royal?' I said. 'I'm not royal. My family is posh, but only because my great grandfather (HH Asquith) was the British Prime Minister.' I could see the publicist in the corner of my eye shaking her head, going 'No, no, no!'. After I finished, she ran over and said 'Don't do that. They love the royal thing in America'."

Her breeding has been a mixed blessing - or so she would have us believe. "If you're not pretty and you're working class" she once told a British newspaper, "you have an easier time in terms of people's attitudes to you". Kathy Burke, the actress who starred in the gritty British drama Nil By Mouth, took the statement personally, and started a celebrity spat on the letters page of London listings magazine Time Out. "As a lifelong member of the non-pretty working classes" wrote Burke, "I would like to say to Helena Bonham Carter, wholly-pledged member of the very pretty upper-middle classes, 'shut up you stupid cunt'.' Bonham Carter replied, politely, muttering something about how her irony had been misrepresented. Then she shut up.

Bonham Carter has made a very good living out of being 'quintessentially English'. "My Mum, who is half-French and half-Spanish, actually gets outraged when I'm called 'quintessentially English'. She says, 'What about our side of the family?' I owe my looks to my Mum. Which was 90% of me getting my first job. And some people would argue 90% of my entire career. Journalists are always calling my features 'Edwardian' or 'Victorian' - whatever that means. I am small, and people were smaller in those times. I'm pale and sickly looking. I look fragile, although I'm not. I look like a doll. But sometimes I just wish I had less of a particular look. A look that was more versatile."

It does sound ungrateful. A woman with a Lux-complexion moaning about her looks. After all, when she was making Margaret's Museum - a film about a coal-miner's wife in Nova Scotia - she looked young enough to get into the movies for half-price. And Yardley approached her to rejuvenate their brand image. "I thought, 'For Christ's sake Hel, don't be so precious. It's just going to be a few photos - mostly for the Middle East market. It's great security'. But it didn't end up just for the Middle East market. It was worldwide. And you're advertising make-up, not what you look good in. So, unfortunately, they put me in bright red lipstick." Which never looks good on consumptive skin. The contract wasn't renewed.

She has always struggled to show her versatility. It's why she took the role of Theresa the drug-addicted doctor in Miami Vice. "Ten days in Miami. I said 'Why not?'. But I was only 19 at the time, and I didn't think I could look like I was old enough to be a qualified doctor. I was taken to meet Don and (Philip) Michael. (Philip) Michael was pleased - major bonding because we had the same birthday. But that was where the bonding ended. The director looked at me and said 'You don't look old enough to be a qualified doctor'. It was a love story, and Don was worried about looking like a paedophile. So they started to put latex on me in make up. You don't start wrinkling at the age of 25, so they gave up and let me get on with it."

In The Wings Of The Dove she plays a chain-smoking Jezebel who seduces men on the subway. The role got her nominated for an Oscar. In The Theory of Flight she plays a woman, crippled by motor neurone disease, determined to have sex before she dies. And in Fight Club, her new film co-starring Brad Pitt and Ed Norton, she plays a death-obsessed, support-group junkie. "But however many modern parts I do - the last three films have been modern - people still refer to me as Mrs Costume Drama. I suppose it depends if the films are successful. So far, the costume dramas have got the most attention. But Fight Club is a studio pic, and I've done very few of those. I've got a feeling it's going to be big."

Bonham Carter wasn't born to act. It was a career choice - when she was five. "I remember this actress friend of the family's" she says. "She was immensely glamorous, and both my brothers had crushes on her. So did my Dad. I thought 'She's got the right idea'. Lots of actors were born to do it. Kate Winslett, for instance. I think she probably came running out of the womb saying 'Where's my mark?' Me, I invented myself as an actor. I'm not particularly emotional. Really. Except when it comes to laughter. I'm squeamish about emotion. I've got a very low sentimentality threshold, and I don't like people showing off. I haven't got that 'exhibitionism of emotion', which is what you're required to have as an actor."

When it transpired that a friend at Westminster school had an agent ("and was a terrible source of envy") Bonham Carter sought representation. At the age of 13. Before she could even spell 'representation'. She ended up as Juliet - in a hi-fi advert. Romeo came out of the other speaker. When subsequent roles in A Pattern Of Roses and Lady Jane Grey drew more critical acclaim than the hi-fi advert, Bonham Carter postponed her place at Cambridge to concentrate on acting. Her next film (the first of four she made based on the novels of EM Forster) was the hugely successful A Room With A View. Like Lucy Honeychurch, the lovelorn romantic in James Ivory's beautiful adaptation, Bonham Carter was an innocent abroad.

"I remember in A Room With A View year, I was asked to present something at the Oscars with Matthew Broderick. I didn't know what to wear, so I just went and got a dress from my cupboard. It was a tulle thing from Miss Selfridge (an inexpensive High Street clothes store in Britain). I shoved a skirt of my mother's underneath it. And tied my own bow on the front. It had flair, I suppose, but looked a nightmare. No-one told me. I even did my own hair and make-up. A friend of my mother's said afterwards 'You looked completely green. Was there something wrong with you?' I remember journalists asking me who I was wearing. They wanted to know the name of the designer. I just said 'The skirt's from my Mum'."

Mum is a psychotherapist. Dad is a banker. A loving couple - and the main reason Bonham Carter took a full 30 years to leave home. She still feels guilty about abandoning her father, who was left in a wheelchair after an operation on a brain tumour went badly wrong. It's why she only moved a short bus ride away, and goes home for dinner whenever she can. "It took me so many years to move out. I'm definitely a bit of a Peter Pan. Definitely a reluctance to grow up. It all seemed really nice at home - why change it? Part of me would prefer not to have any responsibility whatsoever." Now the builders have finished renovating her new place, and it's the swatches of furnishing fabric that are making the mess.

Bonham Carter lives in Belsize Park - one of London's prettier villages, and full of eccentric fashion boutiques. But Bonham Carter's black jeans are from a mall in Paris. And the cardigan looks like it came from a thrift-store. "I'll try on things that look nice, but they're just never comfortable. I've got millions of shoes but I always end up wearing these great clumpy things (she points at a pair of black Buffalo trainers that she's been wearing this past year) because they're comfy and they add a few inches. Unfortunately they don't go with everything. But you get the length of leg (she doesn't like her legs) without the pain. I tend to buy things I would never wear - only because I would like to be the sort of person who would wear them."

She is happy to live alone. But since she met Kenneth Branagh - while filming Frankenstein in 1994 - Bonham Carter has maintained a discrete silence about her private life. She denied their involvement for a long time because, technically, he was still married to Emma Thompson. Then they were photographed kissing in a park. The pair remain tight-lipped, even though their affair is now rumoured to be over. "The press know we haven't really ever spoken about it, so all they get from me is 'Not going to go any further'" she says. "As soon as you begin to have a dialogue with the press, it's an invitation to ask. It's a very subtle line. Before you know it's a licence to hang around outside the house. As it is, we've had remarkably little hassle."

She resolutely refuses to be imprisoned by fame. Her friends say that she has stopped noticing when people stare at her. It certainly helps that she's short-sighted. "Famous people come up to me, but I don't know who they are because my sight is so bad. It's always at the pool of the Four Seasons, when I don't have my lenses in. And my glasses are in my room. The last time it was 'Hi Helena'. I could tell he was Caucasian. I said 'Hi'. He said 'How are you doing?'. I said 'I'm fine. But I can't see who you are'. It was Matthew Broderick. The next day, the same thing happened with Sigourney Weaver. The number of people I ignore, I'm lucky I work at all in this town."

Branagh is convinced that Helly (as he calls her) will always get work. "It sounds like a silly thing to say of a young person, but I sense a growing admiration for her longevity. I imagine it's quite hard for people like Helena - people who are truly learning about what they do in the spotlight. The advantages of having such a significant success at a young age (A Room With A View) are that you have opportunities that other people wouldn't have. But there are disadvantages too. You're more exposed, and don't have the chance to make mistakes quietly in a career that starts off with less noise. People really admire the fact that she has stuck at it, and dealt with all of that prejudice. I think she's finally paid her dues."

David Fincher, the director of Fight Club, agrees. "Brad and I were watching The Wings Of The Dove, and thought if you took Helena's face, and it wasn't in some period garb, it could really work. And what an actor. She's incredibly physical. She's not an intellectual actor, which is surprising, because in the past she's played such restrained characters. Marla (her character in Fight Club) has her chin thrust forward, with her head down. When Helena came on set you could see she was either ready or she wasn't. When she was ready, no matter what you said, she wouldn't look at you - she was in Marla mode. She was like this little train - burrowing along with these puffs of cigarette smoke trailing behind her."

Fight Club was written by Chuck Palahniuk, a Portland diesel mechanic. He jotted down plot threads while he was fixing trucks. "It's a real men's picture" says Bonham Carter. "It's about two young men who set up an amateur bare knuckle fighting club for disadvantaged youths, and the woman who comes between them. It could be described as Raging Bull crossed with Harold and Maude. But I'm in the Harold and Maude bit, you see." Bonham Carter plays Marla, who meets Jack, Ed Norton's character, at a support group for the terminally ill. He works out very quickly that she's a fraud. But he's a fraud too. The pair are both hooked on the support group culture.

"Ed and I tried to rehearse in a conventional way" she says. "But it all fell apart. Ed and Brad would always be playing basketball. The whole film is about boys being boys, and there they were playing basketball. When I suggested perhaps they could stop playing - because I don't actually play basketball - they were 'Oh sorry, sorry'. Ed talked a lot on set, and Brad took pictures. Brad just looked like a God, in three dimensions, but was nauseatingly normal. A lot of the time I didn't really understand what he was saying. He speaks in this street voice. I don't know where he picked it up. Fincher obviously understood him, but they had both worked on Seven. I think it's his own Brad language."

Polly Steele, the producer of Bonham Carter's next film, Women Talking Dirty, was surprised at how thoroughly un-Edwardian she was. "It's just that I've seen her in so many contained roles" says Steele, "that I didn't expect to find her such an ebullient person. That's so unfair, because all I've ever known of her is period roles with corsets and heaving bosoms. But she so isn't like that in reality." It's why Bonham Carter is now confronting the misconceptions in - of all things - a watch advert. "The advert has a picture of me in period costume" she says. "And a picture of me as a modern woman. It says 'I am what I am. I'm not how others perceive me'. So I'm exploiting the misconceptions. For once, I'm making a lot of money from them."

The release of Bridget Jones' Diary - which is scheduled to be made into a feature by the producers of Four Weddings And A Funeral - will change the misconceptions for good. Helen Fielding, the author of Bridget Jones' Diary, wants Bonham Carter for the lead. The film starts with Bridget's New Year resolutions to quit smoking (Bonham Carter once gave up for six days), lose 10 pounds, and develop "inner poise". Bridget is a disorganized, insecure, weight-obsessed woman, who can argue the pros and cons of nicely-pressed sheets. She is a thoroughly modern role. Along with Fight Club and Women Talking Dirty, the part might allow Bonham Carter to pack away the corset once and for all.