Kate Bosworth: On the crest of a wave

Blonde sweetie, porn star's girl, the virginal Sandra Dee - who will Bosworth play next? She talks to Charlotte O'Sullivan

With a client list including Russell Crowe, Kate Bosworth's agent is a busy woman. And today her plate seems especially full. She may have to sit in on our interview. She wants the chat to steer clear of Bosworth's current paramour, Orlando Bloom. And she needs me to sign something, to ensure that my piece won't appear in the tabloid press.

Under my breath, I point out that Independent readers don't give a monkey's about Bosworth's relationship with the LOTR pixie and that our discussion about Wonderland - Bosworth's new film - is unlikely to contain anything of interest to The Sun. True, the 21-year-old actress plays the girlfriend of the real-life Seventies porn star John Holmes (Val Kilmer). But I won't be asking whether Val has a big one...

Strange that wispy-wee Bosworth should be creating such a stir. In 1998, she was just another fresh face, notable for her debut as Scarlett Johansson's friend in The Horse Whisperer (which she secured because she really can ride). The next project of note was the girl-surfer movie Blue Crush (2002), and suddenly she'd acquired her own little cult, with her David Bowie eyes (one hazel, one blue) the focus of special worship.

The question is where she goes next. Last week saw the UK release of the rom-com Win a Date with Tad Hamilton!, a blatant attempt to crown Bosworth America's new sweetheart. (Her character is a church-going virgin; watch that blonde pony-tail swing!) Next week, there's Wonderland, a tense, splintered indie movie about one of the most brutal mass slayings in LA history, in which Bosworth - among other things - gets pimped and beaten up and smokes crack.

After that, there's Beyond the Sea, Kevin Spacey's (apparently) very unglossy biopic about the Fifties pop idol Bobby Darin, in which Bosworth appears as Darin's first wife, teen-queen Sandra Dee (a woman who suffered from anorexia for most of her life). Such a choice suggests, at least, a certain ironic detachment on Bosworth's part. So what's the deal? Is she Dee to a T, or something altogether more edgy?

Bosworth tiptoes into the room (without her agent), clutching a cup of tea with both hands. "Hello," she says in a quavery voice. "I've got a cold..." I say she looks very healthy - and she does, radiant even, and much more interesting than on screen. Her eyes are huge; her freckles are, too, swarming with particular keenness around her Cheshire-cat mouth.

The wideness of her face makes you think of Michelle Phillips (of The Mamas and the Papas fame) and that other Sixties icon, Tuesday Weld - perfect pretties with a kinky chink in their armour. "Healthy?" she says, rolling her eyes and tugging at a strand of straggly golden hair. "Well, thanks. I guess I don't look as bad as I feel."

We talk about Wonderland, and she's soon gesticulating vigorously. Her character, Dawn Schiller, was 15 when she met Holmes (he was 32). As his drug habit spiralled, she was increasingly used as his punch-bag. In the film, we see her in the bathtub after one encounter, rigid with shock. According to Bosworth, "Dawn was kind of catatonic because she was afraid that if she moved or cried, he'd leave her." Her eyes blaze: "I can't fathom that."

It's not that she's "above" dysfunction. As she says: "Everyone knows that feeling. You know, the guy seems an asshole, but when he's with you he's really nice and you're telling all your friends, 'When he does this, he's really nice.' That idea of the bad coming with the good - you can really talk yourself into that."

But the Dawn-Holmes dynamic was too crazy. "There's no way I would have stayed in that relationship, that's for sure." There's a scene where the notoriously explosive Kilmer, out of the blue, started screaming at her "in character". She laughs. "I've never had a man yell at me that way."

Asked about the reaction to the film in the States - that the characters were too "unlikable" - she pulls a wry face. "You know what? I don't really read the critics. I think of a good film as one that makes me go, 'Wow, that person's really interesting.' Or really screwed up. I feel like Europeans are more open to enjoying film for the experience. Americans sometimes need to 'relate'. There's this desire to see the world in a certain way, to be friends with the characters, rather than just..." A big sigh. "You know, Casino is a great film. And no one in it is nice."

That said, she herself can sound "nice"; very much the innocent, unworldly ideal. Her favourite book is The Little Prince, and James Dean - another Antoine de Saint-Exupéry fan - is her favourite male film-star. Sometimes the sweetness can feel contrived: just as often, it's endearingly, giddily natural.

The crack-smoking scene caused a few problems. "I'd seen Val do it in a scene before, and I knew you had to light it underneath and let smoke form, but I didn't really know when to take a hit. I didn't want to say anything, because I didn't want them to think I was kind of stupid..." and her voice tails off. "But all of a sudden I just put it down and said, 'I don't know what I'm doing." She giggles. "And everyone on set had this huge sigh of relief. Like, 'Thank God, she's not a crack addict!'"

While Kilmer was method-acting up a storm (wearing Holmes' actual wedding ring round his neck, covering his trailer in photocopies of bloody hand-prints), Bosworth used less violent tactics. "Usually I talk to my parents at least every other day, just to check in, 'cos I'm an only child, and I love them..." But she didn't ring during the 21-day shoot. "I told them, 'I need to talk to you less - I need to feel a bit lonely.'"

"You know what?" she says excitedly: "I think Dawn saw John as a kind of father substitute. I love psychology. I'm hoping to study that in high school - in, um, college." What would Freud make of that slip, I wonder? Bosworth has been accepted by Princeton, but keeps putting it off.

She says she finds so much stimulation in acting, getting to live different lives. Working on Blue Crush, she became "a real surfer type. I was really into that laid-back thing, getting fit, leaving my hair loose... If you met me while I was on Blue Crush [she leans back on the sofa] I'd have been, 'Hey...' Right now, I'm not doing anything, so you can meet the real Kate."

The PR pops her head in: the photographer's arrived. Bosworth nods: "I need a touch-up." But she looks lovely, I protest. And sort of perfect, given that Wonderland isn't a "touched up" sort of film. "My poor lips - I've been picking at them while you were sitting there," she says. "That shows how comfortable I am with you!" I muse on why it's OK to feel comfortable with a journalist, but not a photographer. Ten minutes later the real Kate appears, looking like a Vaseline angel, lips and hair aglow.

The agent's back. So are three other people I haven't met. (Bosworth says she's got a "very cool" team. Maybe this is them.) The photographer starts snapping, and the agent rushes over. "Just a second, a bit of hair's popped out." The strands are poked back in and the photographer asks Bosworth to move her arm - casually. She does so with a polite smile, but it's no use. Right now, she makes the Stepford wives look floppy.

Through some kind of hair'n'make-up miracle, there are now five people at the back of the room. The new face rushes over. Bosworth's blonde mane, it seems, now lacks volume: he poufs it up, and her team buzz with relief. Finally, Bosworth comes over to say goodbye. She says I should look out for Beyond the Sea: "I think it's going to be really good. That couple - they weren't what they seemed. We go into Dee's depression, her alcoholism."

On Wonderland, Bosworth got to know the real-life Dawn Schiller, who she says is "beautiful, very 'young in spirit' [Bosworth makes the quote marks]. She collects fairies, you know? That's what I found so intriguing. She was the most impressionable person in that group, the youngest, the most naive, and yet she's the one who survived."

Did she meet Dee (who turned 62 last Friday)? "No. Actually, she's a very lonely person. It's sad. She married so young, but it didn't really work out. And her career had pretty much died by 1967. You know, she was 26 and it was all over. I'm trying to remember this thing she said about herself, that she was a 'has-been and a never-was.'"

Spacey, rather chillingly, says Bosworth was perfect for the part because "she had, in her own personality, so many qualities that were dead-on for Sandra Dee". So where does that leave her? Will she, like Dee, be one of those super-hot properties who looks good, feels lousy, and then disappears? Or will she - this smart yet child-like creature - prove the ultimate survivor?

I watch Bosworth a few days later on Jonathan Ross's chat-show, giggling, in a stricken sort of way, at his tirade of gags. Maybe getting away from this business would be a good idea. Maybe spending three years in high school - I mean college - would give this pretty young thing a chance to breathe.

'Wonderland' is released on 7 May