Helena Bonham Carter surprised many by transforming herself from a quiet English rose, appearing in period dramas, into a dangerous vamp living life on the edge in Fight Club. Now teamed up, personally and professionally, with director Tim Burton, she is facing her toughest challenge... that of doing what her boyfriend tells her to

THE moment you set eyes on Helena Bonham Carter, you can't help but be drawn in. An actress with a timeless aura about her, she's seemingly as fresh as the day she sauntered on to the screen as the delicious Lucy Honeychurch in A Room With A View. While her skin still has an innate purity to it, it's her almond-shaped brown eyes, both mournful and full of curiosity, and her lips - ever ready with a wicked smile - that will get you. Of course, you can't help but study the nest of hair. Balanced on top of her head with gravity-defying grace, it's little wonder one interviewer recently noted she looked like "a cross between Bo Peep and Marge Simpson". Then there's the well-heeled accent, occasionally pronouncing "yeah" as "yah" in a way that's so Eighties, it's almost fashionable again.

I can't put it better than Paul Bettany, her co-star from The Heart Of Me, when he dubbed her: "barking mad, keen as mustard and funny as f**k". Madness does seem to be a recurring theme in her work, though she comes across as quite sane. If Bonham Carter's eccentric, in that it's only in a delightful, charming way. Her fashion sense - all fluffy cardigans, frilly blouses and crumpled ankle-length skirts - is so idiosyncratic that you may have trouble deciding if she's retro-chic or just simply a bag lady made good.

Now 40, the woman once nicknamed "little thing" by the Merchant/Ivory troupe she began her career with has finally grown up. Six years into her relationship with director Tim Burton, the prince of darkness behind such films as Sleepy Hollow and Edward Scissorhands, she finally seems to have found her soul mate - and not just because, as she once joked, "we both don't believe in combs".

They evidently share the same quirky outlook on life, typified by the fact that when they first met, rather than move in together, Burton bought the house next door to Bonham Carter's home in London's Belsize Park and knocked it through. They now share this extended living space with their three-year-old son Billy Ray, who is evidently being tutored in the way of his parents' Gothic tastes. His mother lets slip that recently she let him watch Harry Potter And The Prisoner of Azkaban - in preparation for her own role in the forthcoming fifth film of the franchise. "It was the one where his aunt blows up into a huge balloon and floats out of the window," she giggles. "That was really traumatic!" She then realises how this comes across. "It was a mistake," she says, more sombrely.

Right now, Bonham Carter is "having a really nice life". As insurance, she knocks on the wooden table that divides us in the London hotel room where we're ensconced. For starters, she only works when she wants, "doing things that are different and well-written". Her latest film, Conversations With Other Women, is just this. Pitting her opposite Aaron Eckhart - they're known only as Woman and Man - they play a once-married couple, now divorced, who reunite at a New York wedding and immediately fall under each other's spell.

"I think she's still not quite clear why she broke up with him. There's a lot of hurt involved, but it ended so badly, they never really discussed it. There's a question mark over the relationship, a What if?' And then she's reminded basically what a pain he is. I think it's what in America they call closure' that she's seeking." She pulls a face. "I hate that word."

The film's novelty is that it's entirely split-screen, much like Mike Figgis' Timecode, so that even when the actors are in the same room they stand divided. "I'm on one side, then if you get bored of me you can look at Aaron on the other side," laughs Bonham Carter, who enthuses over this experimental conceit. "I thought the style would upstage the content, but it doesn't," she says. "It's really interesting to watch."

Director Hans Canosa uses the concept well, at times allowing flashbacks from the couple's past life together to play on one side of the screen in parallel to the contemporary action. The set-up for this virtual two-hander meant that both actors had the camera on them at all times. It's as close as she's ever got to acting on stage. "It had a sort of energy as if we were doing it live," she agrees.

Shot in just 13 days - the fastest shoot Bonham Carter has ever enjoyed - "it meant there was more time acting than waiting, which is often the opposite case with most films". She evidently preferred the experience to working on the Harry Potter film, The Order Of The Phoenix, in which she gets to play the "mad, sadistic witch" Bellatrix Lestrange.

"It was the other extreme," she says. "That was six weeks and I had about five lines you just camp there and every so often do a bit of acting in front of green screens." Given the dotty impression she makes, I wonder if it's this that led to her Potter debut. "I don't know," she muses. "I obviously do madness quite well. I'm not sure. I've never done sadism, actually."

Until now, that is. When we meet, Bonham Carter is in the final throes of finishing off Sweeney Todd, Burton's big screen version of Stephen Sondheim's musical about the murderous London barber. With old Burton favourite Johnny Depp in the title role, Carter plays his accomplice Mrs Lovett. "She's bonkers," laughs Carter. "She makes pies out of human hearts!" It's her fourth live-action film for Burton, though as she says, "it's unlike anything any of us have shot. People keep on breaking out into song!" Does she have a good voice? "I don't know," she says, a little timidly. "Obviously it was good enough for the part. I surprised myself. I didn't think I could sing. I sort of thought, F**k it! I'm going to go and learn to sing and try and get this part.' Apparently I have, but I'm not one to judge, I'd say."

Well aware that most actresses will be jealous of her frequent parts for Burton, silently accusing her of sleeping her way to the top, Bonham Carter is keen to stress this isn't the case. "It can't be a given. He does make me audition. I have to doubly prove that I'm right for it. But there's a sexy formality to it. He'll come to me and say, I'll be very honoured if you would consider playing the part of' This is two weeks after I've done the audition - there's nothing said at the dinner table." This time, the process - which included trying out in front of Sondheim himself - was the most rigorous she has ever endured. "It was merciless," she sighs. "It was really quite stressful. I wouldn't recommend it for any relationship!"

They first met on the set of Planet Of The Apes, Burton's much-maligned 2001 "re-imagining" of the Charlton Heston classic. Cast as an ape, in one of several attempts by Bonham Carter to distance herself from her Merchant/Ivory image, she maintains they didn't get together during the shoot, pointing out he never even saw her face for five months due to the simian prosthetics she was forced to wear. "It was genuinely after," she says. "Everyone thinks that everyone gets together on movie sets. It was when we were selling the movie some months after. We went to dinner with my brother anyway, it's a long personal story that I don't think is particularly appropriate." She tails off for a second, aware she almost said too much.

She reunited with Burton for the 2003 fable Big Fish, which dealt with the relationship between a son and his dying father. She played dual roles, including the first witch of her career, but the release of the film was overshadowed by the death of her own father, Raymond, who passed away two days before the film's UK premiere in 2003. A former Merchant banker, he had been paralysed since Bonham Carter was a teenager, after a routine ear operation had left him unable to walk and semi-blind.

"It had a real resonance, the film," she murmurs. "I knew Dad would love it. And it was really cathartic. Me and my brothers were sitting there. It might seem a bit of a travesty, as if you'd been up close to the real thing, you might think it was a bit sentimental. But it really stood up and it ended up being really healing - he'd been ill for so long, Dad. He really wanted to die."

Bonham Carter followed this with a small role as Mrs Bucket in Charlie And The Chocolate Factory, as well as voicing the lead in Burton's ghoulish stop-motion animated tale, Corpse Bride. "Working together always has its stresses - and joys," she says. "It's always a surprise. I think he forgets how much I talk. And I forget that I basically should obey him at work. I don't obey him at home, so I forget that I have to at work." Yet she admits her relationship with Burton and becoming a parent has helped her settle down and grow in confidence. "As everyone says, it changes your perspective and proportions. It makes you much less serious about yourself. You take yourself less seriously." So did she suffer from this? "I think without meaning to, yeah, over the work."

If motherhood has made her more relaxed - "there are a lot of good things about it," she confirms. "Only good, really" - it evidently helps that they don't live in Hollywood. "Definitely," she nods. "No, don't like that place. Neither of us do. He hates it - even though he was born there, he couldn't wait to get out, I think. LA is so boring. There is a monotony to it."

She says that Burton loves Europe. "I think he's always felt more at home there - well he is now, so he better!" Telling me he "does a very good posh accent now", she thinks her man should make the honours list for employing so many British cast and crew on his films. "Then I could be a Lady," she continues, warming to the idea, "and Billy could be the half-honourable Billy Burton."

Were this ever to happen, it might finally stir some truth in the myth surrounding her family heritage. Despite being the great-granddaughter of former PM Herbert Asquith, and a member of a far-reaching Liberal clan, "I've always been made probably into a bit posher than I am," she says. "That always used to bug my dad - I mean, we didn't have an ounce of blue blood." Landed gentry, they were not. "We don't have country houses. We don't have any family pile; we were poor, relatively." Then there is the fact that her mother, Elena, is actually half-French, half-Spanish. "It wasn't a tremendously British household I was brought up in. That's the irony. I don't even look that British, I don't think. I'm definitely the chip off my mother's block. I don't look like the English side of our family."

If the image of Bonham Carter as the quintessential English rose suddenly seems blighted by green fly, she seems to care as little for this as she does for the family business. "I've never been a particularly political animal - not formally, not party politics. My cousins are - that's just them." Neither is she a feminist, she says. "I mean, not at the moment. You never know but not particularly now" She lets out a self-mocking chuckle. "I tend to rail against any kind of injustice."

The idea of acting, she explains, entered her head at the tender age of five, after she encountered Lisa Harrow, a family friend who trod the boards. "She was so beautiful and glamorous, and all my brothers fell in love with her. So I thought, That would a good thing to do!'" She also cites seeing Judy Davis in My Brilliant Career as a seminal moment. "I wanted to grow up and be her! I was 14, I think. It made a big impression."

As it so often does, luck still played an enormous part. Back in 1985, her picture appeared in a feature in Tatler - a last-minute replacement for her cousin Virginia. Spotted by director Trevor Nunn, she was cast in the title role of a film about Lady Jane Grey in the same year she appeared in A Room With A View. Offers began to pour in, though she also sat an entrance exam for Cambridge. "I did walk around thinking I've got to go to university'. I was very serious, wanting something to fall back on. I was doing the adult thing. Meanwhile, my Dad said, Go to university later. If you have a break, and you can't manufacture this thing, carry on - have a life'. My parents were always very accepting of what I wanted to do."

Following A Room With A View, she became a poster-child for EM Forster adaptations - shooting Howard's End and Where Angels Fear To Tread back-to-back - and developed a penchant for period pieces. Naturally, she dabbled in Shakespeare (as Ophelia in Franco Zeffirelli's Hamlet and Olivia in Nunn's Twelfth Night). She tried her hand at Henry James, winning an Oscar nomination for her role as the scheming Kate Croy in The Wings Of A Dove. And not unsurprisingly, she appeared in Frankenstein, the adaptation by her former boyfriend Kenneth Branagh of Mary Shelley's Gothic classic. But contemporary work was hard to come by. She did it with Woody Allen, playing an unfaithful wife in 1995's Mighty Aphrodite, but felt uncomfortable with the experience. "I didn't really have much contact with Allen when we were filming," she sighs.

If one film changed our perception of Bonham Carter, it was 1999's Fight Club, David Fincher's scabrous assault on consumer culture. Cast as Marla Singer, a woman whose heart appeared as black as her eyeliner, it banished her petticoat image in one moment. "You always know what films will last from what you're recognised for," she says. "And it's still being watched endlessly. A while ago, a woman in her 70s or 80s, in Hampstead, was veering towards me - and you normally think, Ah, Merchant Ivory fan'. You have completely different people for different audiences, but she came up to me and said, I just want to tell you, I so love Fight Club!' I thought, Good for you!'"

You might think this would show her about judging people by their appearance - though she claims to be indifferent to the image she projects on screen. "I'm a bit tired of dealing with it. I don't really think too much; I don't want to think about what other people think about me. All the parts I do, I've never thought, I must change my image'. I only do a part if I want to do it, for my own reasons - if I feel excited by it, or inspired by it. I've never done something to try and change people's view of me. That would be a non-starter." It's why she had no qualms about voicing Lady Tottington for the Wallace and Gromit movie The Curse Of The Were-Rabbit, one film she has yet to show Billy Ray. She suddenly perks up. "I should show him that," she says. "That's a good point."

It's no surprise that such a non-Hollywood couple as Bonham Carter and Burton don't hang out with actors (with the exception of Depp, Billy's godfather) "so much any more". She doesn't look exactly sad at this idea. "We don't actually have that many film friends. It's quite nice to see other people."

If not exactly on the backburner, her career certainly comes second to the domestic bliss of parenthood of late. "I've been doing a bit of the living bit since I met Tim. I suddenly think, Christ, I've lost out on so much!' There's no regret, but now I can catch up. I'm quite happy not working and doing all the nice bits of life." It sounds like she's hopeful for the future. "Yah, yah," she nods. "At the moment."

Conversations With Other Women opens on May 18, Harry Potter And The Order Of The Phoenix is released on July 13