Scarlett Johansson is relaxing, legs stretched out before her on a sofa in one of London’s most luxurious hotels, and pondering the strange phenomenon that is life as a modern-day sex symbol. Think Marilyn Monroe or Sophia Loren transported through time – the hour-glass figure and sprayed-on frocks – and you have some idea of the cinematic heritage of Ms J, a woman who, at just 24, has men young and old (including Woody Allen and just about every other director she’s worked with), praising her as a siren, a muse and a pin-up for our times. Women, too, can appreciate La Johansson. According to one glossy, she possesses the body that most admire.

“Oh yeah, that stuff,” she drawls in that unmistakable, husky voice, raising a pencil-thin eyebrow. “Well, I never expected that kind of thing, to be honest. I think that comes with my age, the whole sex-symbol thing. I’ll grow out of that. It’s a phase. And people want to turn you into merchandise in some way, don’t they?”

If it’s an image that she hasn’t particularly courted, it’s also one that she’s not afraid to exploit. In one of her two new films about to be released in the UK, The Spirit, a visually striking adaptation of a comic book, she’s a femme fatale with the kind of plunging neckline that will ensure teenage boys flock to the multiplexes in their droves, while their girlfriends weigh up the merits of a Wonderbra. “I was inspired by those actresses from the Forties and Fifties, and all that movie-star glamour… you know, that Golden Age of Hollywood. I like that look,” she admits.

Today, her look is more Pippi-Longstocking-meets-Gap: her hair is piled up in plaits, and she’s wearing blue jeans, white and green trainers and a grey slip over a matching cardigan (which is covering a recently acquired tattoo, of a sunrise, on her right forearm). In person, she’s friendly, bright, articulate and supremely composed. If anything, she appears even younger than she is. Indeed, the casually dressed, tiny (she’s 5ft 4in) woman who walked past me in the corridor – followed by a minder twice her size – a few minutes earlier didn’t attract so much as a second glance.

But put Johansson in front of a camera and you see genuine screen presence. She commands the eye in a way that few of her contemporaries do – she was luminous in Girl With a Pearl Earring (playing the maid who inspires Vermeer to his greatest work), and drop-dead sexy in the Forties-set noir thriller The Black Dahlia.

“I think it’s hard to have any kind of perspective on the image that builds up around you,” she says. “It’s funny because I live a quiet life. I do. I turn up for a premiere or a charity event, and then I have my life. And, really, it’s a relatively normal life.”

Johansson has been in the spotlight since she was a teenager. She was born in New York and has a twin brother and an older sister and brother. Her mother, Melanie, is a film and TV producer, and her father, Karsten, a Danish-born architect. Johansson loved acting and singing as a child, and from school plays graduated to TV appearances and minor film roles. Her breakthrough came with The Horse Whisperer, playing an awkward young girl traumatised by a riding accident. Directed by Robert Redford, it announced her as a talented actress with huge potential. But even though she earned good reviews, not even Johansson herself could have predicted the meteoric rise that would follow. She looks back on that performance with a mix of fascination for her younger self and pride.

“That feels like an age ago, and yet it’s also fresh in my mind,” she says. “I was 12 when I made the film and 14 when it was released. It feels like for ever. I’ve spoken to other artists about this and there’s a purity, an ease, an innocence that comes with earlier performances. And although you learn from experience and you grow, there’s something that you never really get back.”

This is a little harsh. Although Johansson was indeed remarkably natural in The Horse Whisperer – and excellent as a 17-year-old in the much darker Ghost World – she has made the difficult transition from child to adult actor with ease. Her best performances have come opposite heavyweight players, too. She matched Bill Murray – no mean feat – in Lost in Translation, in which she played the new wife of a musician stranded alone in a Tokyo hotel, who strikes up an unlikely bond with a middle-aged actor in town to do a commercial. Directed by Sofia Coppola, Johansson was thoroughly charming and won herself a Bafta for Best Actress in the process.

She earned another Bafta nomination for Girl With a Pearl Earring, and even when the films she’s appeared in have been duds (The Island, with Ewan McGregor) or average (The Nanny Diaries), Johansson is usually worth the price of a ticket. “I’ve been fortunate,” she says. “I’ve worked in a lot of really great productions.”

Now the aim is to ensure career longevity. It may seem a long way off, but, in one sense, she is anxious to leave her twenties – and that sex-symbol baggage – behind and get on with it. “I look forward to growing in the industry and ageing in the industry. A lot of actresses take their meatiest roles in their thirties and forties. And I’m looking forward to that. I’m at a little bit of a funny age where I get that ‘sexiest woman’ thing, and it feels like a label for right now. But maybe when I get older, it won’t be like that.”

Not that she’s complaining. Mostly, she seems rather bemused by the fuss that surrounds her. There are stories of a feud with Lindsay Lohan and songs written in her honour by a pop star (Katy Perry’s I Kissed a Girl) she has never met; there’s gossip – flatly denied – of a passionate encounter with Benicio del Toro in an elevator, and one fan bidding $40,000 on eBay to attend a party at her side. “It’s part of the whole media circus,” she says of the relentless tittle-tattle. “But why me and not somebody else? I guess it has something to do with the fact that I’m confident and speak my mind, so I guess I’m labelled in some way or another.”

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