How Keira won her battle

Keira Knightley wafts into the room and, for a moment, I fail to recognise her. Gone are her golden locks, replaced by a sober brown ponytail. When her PR first saw her with brown hair, Knightley squeals excitedly, he looked right through her. "But I was on the Tube the other day and this lady leaned over and said, 'Bend It Like Beckham was great', and I was like, 'Damn!'"

She coloured her hair for the role of Guinevere in the new King Arthur movie and, she reveals: "It's my natural colour - I've been dyeing it since I was
14." She shrugs her narrow shoulders wanly.

If this makes Knightley sound remotely ditzy, don't be fooled. As 18-year-olds go she is surprisingly mature. Her appearance at Monday night's premiere of her latest film, Pirates of the Caribbean, proved the point.

While most teenagers might be forgiven for wearing a recklessly non-existent dress, Knightley turned up in a modest Alexander McQueen top and jeans, looking understated and womanly. "Mind you," she says, mercifully betraying at least a hint of teenager, "I did put 30 friends and family on the guest list!"

Keira Knightley's life, she concedes, "is really great at the moment and I intend to enjoy every minute of it". Since her appearance as Jules in last year's Bend It Like Beckham, her career has taken off with roles in the TV series Dr Zhivago and the forthcoming Hugh Grant offering, Love, Actually.

With Pirates having knocked Terminator 3 off the top spot in America last weekend, things can only get better. "This is just such a surreal world to be in," she says, "and I'm determined to have fun in case it all ends tomorrow."

She's a curious creature - all wide-eyed innocence, coupled with a knowingness I've never come across before in someone so young. She does the giggly teenager bit when talking about her Pirates co-stars, Orlando Bloom ("he's like a one-man boy band"), and Johnny Depp ("I always used to be a DiCaprio girl, but now I'm a Johnny girl"), but then she does the grown-up bit equally well.

Being so beautiful and successful must, you feel, make her universally loathed by her peers. "Actually, they usually just giggle at me. But I heard I was going to be filming Pirates when Johnny Depp was in town filming and my friends literally camped outside his place for two weeks. I couldn't bring myself to admit I was going to be starring with him in a film, so yes," she adds, "my friends probably hate me."

Life, she insists, hasn't changed too dramatically. "When Bend It Like Beckham and Dr Zhivago came out in the States, I was here, so I've dodged the fame side of things. I don't get recognised a lot.

"I didn't get into acting for the celebrity aspect because although my parents are in the business [her father is actor Will Knightley and her mother, playwright Sharman Macdonald] they were in theatre, and theatre is what I'm desperate to do."

When not gadding about Hollywood, Knightley lives in Teddington with her parents. She is coy on the subject of boyfriends, claiming that there is "no one special at the moment" and even more unbelievably that "I've never been chatted up in my life". And although in love with her work, she admits that "If I could have chosen all this to happen at a certain time, I would have chosen it after university.

"I've always been a snob about qualifications and it's the greatest irony that I left school before I could take my A-levels. I'll really feel bad when my friends get their results. And I'm going to regret not going to university. But acting has always been my dream and I have to pursue it while I can."

Certainly, she was a precocious child, asking her parents for, but not getting, her first agent at the age of three. Three years later, though, she struck a deal with them, promising to study every day to overcome her dyslexia in return for an agent.

"I was diagnosed when I was six. I worked hard to get the better of it and by the time I got to secondary school, it was much better. I can read and write - badly - but I'm fine reading scripts. And I did leave school with starred As in my GCSEs!"

Knightley, with no stage school training (she attended a mixed comprehensive), won the part of Natalie Portman's decoy in Star Wars: Episode 1, at 14. "But I was never allowed to miss much school," she adds primly. "It was difficult - because you take time out, you lose your friends and have to wriggle your way back in.

"I did a film called The Hole when I was 16 and, when it came out, a couple of the popular girls at school said, quite loudly, 'She's in a crap movie, so it doesn't count.' It was rude and I was upset," she says, smiling bravely, "but if you let that stuff bother you, you're going to be in for a tough time. I wasn't popular at school. I learned to let those comments wash over me. I toughened up quite quickly."

Tough she certainly is; canny, undoubtedly. It is telling that she caused a rumpus last year when she stripped off for her role as Lara in Dr Zhivago. "I was told I didn't have to do it, but I said, 'No, it's a beautiful scene, we have to do it.' It's extraordinary that you can cause an uproar by getting your tits out."

She gets up to leave, patting her newly brown locks. "I feel less blonde now and, er, smarter! I actually feel much stronger, too," she smiles.

I think she was born that way.

Pirates of the Caribbean: The Curse of the Black Pearl opens on 8 August.