When I travel to Long Island to meet Natalie Portman, she is days shy of her 18th birthday and goofy as can be. Spotting a dribbling, toothless old man in the train station car-park, she nudges me in the ribs and giggles, "Look, it's your boyfriend, Emma." Decidedly unregal behaviour from the girl playing Queen Amidala in the long-awaited Star Wars prequel, but then Portman is a most unprincessy Hollywood princess. Explaining why she took on the central role in the most anticipated film of the decade, she shrugs, "I did it because I wanted to have a fun summer. Going to London, working with Ewan McGregor, Liam Neeson and George Lucas just sounded like the most fun way to spend my 16th summer."

Portman is a big rib nudger, food dribbler and face scruncher. This is surprising, since everything she does on film is so delicate. Her forte is stillness. In common with most great film actors, she has a face of profound expressiveness. From Luc Besson to Lucas, Portman's directors have found that they need only focus on her face with its shiny, dark eyes, full mouth and skin so pale you can almost see the blood moving beneath it, to bring an audience to tears.

When she first meets me off the train, Portman is loth to make eye contact. She is self-conscious about a huge red welt on her nose, the result of an experiment with a moisturiser intended for much older women. You can see how she might get confused. She has been playing a grown-up since she was 12, when she made her debut in Luc Besson's Leon.

The underworld waif, Mathilda, sealed Portman in the public image as equal parts Jodie Foster and Brooke Shields. Like Foster, her talent was clear from the outset. Like Shields, she had to deal with being dubbed, before even entering her teens, "the most beautiful woman in the world"--something she had difficulty with, coming from a family of Jewish intellectuals, where books were always emphasised over looks.

Sitting, entirely unrecognised, in the station's greasy-spoon cafe, she explains, "I've always tried to separate my looks from all the other aspects of myself. I think girls are taught so much to focus on their looks that they tend to have their personality and intelligence develop slower than boys. As an actress, you have to recognise that it is not necessarily in the best interests of the character for you to look your best."

She cites, as an example, Susan Sarandon, with whom she has recently completed filming Anywhere But Here, based on the acclaimed Mona Simpson novel about a dysfunctional mother-daughter relationship, and due for release later this year. "Maybe it's because Susan is so beautiful that she isn't embarrassed not to be."

Conveniently, at this exact moment, Portman dribbles a bite of food down her front. She had her wisdom teeth removed a few days ago and struggles with her scrambled egg. She can't be much more than 5ft and 80lb, and like the true movie icons, in real life her head seems too large for her body. Natalie Portman is not the actress's real name. That remains a well-guarded secret. Until a supermarket tabloid printed it last month, the media didn't even know where she went to school. Her friends aren't bothered about her fame. She waves to several as we drive around. But it wasn't always that way.

"I was really popular up until 13, really outgoing and bossy, and never worried about seeming it. Then I came back from shooting Leon, and no one would talk to me. Everyone kept saying, 'Oh you think you're such a hot- shot.' I had no friends. It was the worst year of my life." She changed schools, opting to go to a public (that is, state) school, and made plenty of new friends. "The only thing is, I've been much more inhibited. Ever since that experience, I make an extra point of not seeming arrogant. I used to be the first one to get on stage at school and sing, and now..." she gazes sadly at her gloppy scrambled egg, "you couldn't pay me to do that."

After Leon came Beautiful Girls, Ted Demme's New England saga of male-female relationships. Portman, then 14, played Marty, the underage object of admiration and lust for Timothy Hutton's 29-year-old character. Mira Sorvino and Uma Thurman co-starred, but men left the film destroyed by their reaction to Portman. She is smart, she is beautiful. She is 14.

Although the Hutton-Portman romance is never in any way consummated in the film, Portman's performance was the first time that a Lolita type was portrayed as having real intelligence, self-will and depth of feeling. Ironically, Portman herself had yet to experience even her first crush. "That happened right after filming ended. And it was so hard, because I was still in the Beautiful Girls mindset, and it made the crush so much more difficult."

Her only uncomfortable film moment to date is in Woody Allen's Everyone Says I Love You, which followed her acclaimed performance in Beautiful Girls. Playing one of Goldie Hawn's privileged Upper East Side daughters, she was, frankly, bad. "Oh God, I was so awful in that. My agents said, 'Well, you just don't turn down Woody Allen.' But I was so wrong because there was so much improvisation and I just couldn't do it."

Portman has always played her age and has yet to take on an adult role, because she doesn't yet feel like an adult. An only child, she lives with her father, a fertility specialist, and mother on Long Island, has to ask permission to go out or borrow the car, and has less money to spend than her girlfriends who have Saturday jobs in shops. "My parents have been very good at separating that. They don't let me use any of my earnings. They pay for everything in my life. They'll pay for college for me. They want to feel like they're still taking care of me." She has already been accepted early by both Harvard and Yale.

However, the most intense period of change in her life, she says, was her 16th year, when she took on the role of Anne Frank on Broadway. Having played Anne every day for almost a year, she is fascinated by the most recent edition of the diary, including the segments criticising her mother, that Anne's father had previously edited out.

"The parts her father excised were because he didn't want his wife's memory tarnished." says Portman. "But it was just the age Anne was at, and if she had been able to live two years past that, she and her mother would have gotten along fine." She responds angrily to a New Yorker article decrying the play's "Americanisation" of Anne, and Portman's portrayal of her as happy-go-lucky, as if Anne's Jewishness were the least relevant factor in the story.

"I found that kind of strange. I am Jewish and I was born in Israel and my grandparents escaped from concentration camps, whereas my great- grandparents didn't." Portman is, in fact, one of the first movie stars to be open about her Jewishness. Although Jews, irrefutably, do run Hollywood, they have never been keen to see themselves on screen. Whereas Lauren Bacall struggled to hide her background from Howard Hawks and Winona Ryder changed her name from Horowitz, Portman has gone on record as saying that reading from the Torah is more nerve -racking and exciting to her than making films, which makes the New Yorker's allegations more hurtful to her.

"Some people were weirded out by the fact that we made her this sparky little girl. But when we talked to members of her family and to Miep Gies [who helped to hide the Franks], the one thing that they all told us was how annoying Anne was; that she was always running around and making noise, and people said that they were found because she was so noisy, which had always been their greatest fear. She was very curious and always asking questions and just bothering everyone pretty much. To write that whole big article is such a self-important thing to do when it is so important that people know this story."

We talk about Anne's desire to be good, her fears that she twists her heart around so that the good is on the inside and the bad on the outside. "I do interviews all the time. You want to be good and positive, but you can't give too much away or you feel like you don't own yourself. It's a second struggle on top of that first struggle to maintain your goodness and inner world, and maintain your belief that other people want to be good. Especially when you see how you can be treated as a young person."

When I ask her to elaborate, she sighs. "I've been treated badly by producers and I definitely speak up then--the way they make me work, the conditions. I am not difficult at all and I'm a really hard worker, but I have had some very high demands made of me. People have wanted me to break the law by overworking. Breaking child labour laws means they lose less money. But those are laws for a reason."

Her experience on Star Wars, she says, was--apart from the Tunisian desert heat- -a pleasant one. Her Queen Amidala (the mother of Princess Leia) is less a love interest than a mother figure. Like Carrie Fisher before her, she is the Wendy to Anakin Skywalker's intergalactic Peter Pan, looking after him and the other Lost Boys. Signed up for the next two Star Wars episodes, she will be playing the role until she is 24 years old. "I thought it was a good way to grow up in films. I get to have the romance, and then having kids... always playing my age. If I do decide to continue in film, it's a great way to make the transition to adult roles. And if I don't--which is definitely possible--it would be a nice last thing to do."

Amidala appealed to her because she got to use her imagination, inventing an accent and staccato style of speech which, like the film itself, has the critics divided. She is not unduly worried. The Star Wars movies, even when they first came out, got mixed reviews. The reviewers have always been impressed by the technology and unimpressed by the dialogue and characters. "George always said that he's making these films for I0- year-olds." She stabs the copious remains of her scrambled egg and laughs, "I think people need to calm down a little bit."

She is now about to start filming in Texas on a love story called Where the Heart Is. She was the first actor signed up and reports that this is the first time she has felt that filmmaking is an industry rather than an art. "Remember, I had this fabulous first experience making a European art movie with Luc Besson, who is an incredible genius and maverick. And now I'm watching the way a big studio operates. With the main guy, it's very clear that they care more than just 'Is he a good actor?'" The one thing Portman would really like to do is a John Hughes-style teen comedy, but she doesn't get offered them. Like Jodie Foster, she is perceived as so talented an actress that she would not be accepted in a light-hearted comedy, which seems absurd, especially after you've spent a morning in her infectiously silly company.

The interview is finished, but the train back to Manhattan doesn't leave for 40 minutes. "No problem," says Natalie. "We'll think of something to do." But there is a problem, which is that there isn't much of interest around stations, especially on Long Island. Portman decides we should drive out to Huntington, where she is going to show me the sailing boats. But we are quickly distracted. Walking through the park, towards the bay, we spy a large slab of rock adorned with the words "Billy Joel". Aware that Joel hails from Long Island. we still can't work out whether it's a gift from him or an abstract representation of the piano man. I take a Polaroid and she ducks into the shot, thumbs aloft, grinning manically.

We drive on out to a vast drugstore, the kind that stocks the trashiest magazines and cheapest make-up. Wandering around, we spy an entire aisle devoted to Star Wars merchandise: tea towels, paper plates, stationery. Portman discovers a toothbrush with her face on it. She picks it up and, rather alarmingly, begins to brush her teeth with her own face. Suddenly, she is distracted by a rack of flavoured lip glosses. "Ooh", she shrieks, "kiwi strawberry!"