Jessica Alba has been hard to miss this summer. Looming 30ft high on hoardings and posters from Sunset Strip in Hollywood to the Croisette in Cannes, her toned body and golden hair have been a key feature of the campaign for Sin City, in which she plays Nancy, a young woman with a heart of gold in a city of dark desires. And now? There she is, blocking out the horizon again, in the campaign for another lavish comic-book-to-screen adaptation, Fantastic Four.

The studio marketeers love to trade on her sex appeal, unashamedly inviting a male audience to come closer. But when you do, you find there’s much more to Alba than her exotic appearance.

She is a razor-sharp 24-year-old who appreciates that while film studios are paying big bucks for her looks, she has to deliver some pretty smart acting to make them come back for more. “What happens when the looks fade?” she questions. “If I don’t establish myself as someone who can act a part rather than look the part, I will soon be finished.” There is no sign of that yet, as Alba has reinvented herself several times in an 11-year career.

She moved from her native America — her head-turning looks come from a distinctive cocktail of Danish-French mother and Mexican father — to Australia, at 14, with her mother as chaperone, to appear in 44 episodes of the television series Flipper. Then she became the smart-ass teen in a succession of forgettable films.

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But it was back in television, playing the part-feline, part-human title role in Dark Angel, that she really came into her own. It was masterminded by James Cameron, the director of Titanic, who made sure Dark Angel sold all over the world to establish Alba as both a face and a name.

We are meeting now at the point of yet another gear shift. The television series is over and Alba is back on the movie market, with her reputation much enhanced. After playing the title role of bar girl-turned-music star in the moderately successful Honey, she has struck gold with the stylish and darkly absorbing Sin City. When the director, Robert Rodriguez, flatly refused even to consider her for the role of Nancy, the sweet heroine forced to dance in a bar, she took action.

“I went after it,” she says. “His reaction, even then, was, ‘But she’s this tough, dark, Latin chick. She can’t play this innocent, pure girl who doesn’t have a bad bone in her body.’” Alba dyed her hair blonde, offered herself for a video-tape audition with the casting director — “The first time I’ve had to audition in five years” — and was finally summoned to meet Rodriguez.

“I told him, ‘I can do this. I am an actress. That’s the point.’” She made enough of an impression to be hired. But he took revenge — of a sort — when refusing to give his star choreography for a sexy lasso dance. “He just said to me, ‘Did you ever see Salma Hayek do a snake dance in my film From Dusk Till Dawn?’ I told him I did, and that it was the sexiest scene I’ve ever seen. He said, ‘Salma did not need a choreographer for that — and neither do you.’ So I was left to rehearse myself, stripped down to my underwear, in front of the full-length mirror in my hotel room. I thought, ‘You asked for this role: now show that you can do it.’”

Alba delivers a powerful combination of looks and determination, plus a nice line in acerbic wit. She dresses for our interview in what can best be described as a tiny slip, with thin straps that fall carelessly off her shoulders, revealing an expanse of flawless, caramel-coloured skin. Her blonde hair accentuates her brown eyes. She is, in a word, a stunner. But she clearly knows what she is doing; she also knows how to make friends and allies. “You have to want to do this job more than anything,” she says. “You also need constantly to remind yourself, ‘It is only acting.’ Too many people in Hollywood go insane. I aim to keep my sanity.”

She seems to have kept both feet on the ground while effortlessly taking her next big stride in Fantastic Four. This time, she plays the female lead, Susan Storm, in another comic-book caper, alongside Britain’s Ioan Gruffudd as the astronaut Dr Reed Richards. It is a slice of geeky nonsense, in which Richards takes his spaceship, with Storm, her young brother (Chris Evans) and his best friend (Michael Chiklis) on board, into cosmic radiation that genetically transforms the crew. Their DNA is irrevocably altered. Richards is suddenly able to stretch and contort his body into any shape he can imagine, becoming Mr Fantastic. Alba’s Storm can disappear while projecting powerful force fields and is hailed as the Invisible Woman. Evans’s character can engulf his body in flames and take flight at will: the Human Torch. Chiklis goes one better as a rocklike superhuman known as the Thing. Thus, yet another battle begins: the Fantastic Four against all earthly evil.

How much of an appetite audiences have left for films based on Marvel comic books is, of course, another matter. But 20th Century Fox has put the full weight of its marketing expertise behind a simultaneous launch across Europe and in America, and predicts a big hit. Alba seems unmoved by the high level of expectation. “It was a giant Hollywood movie, with so much financial backing,” she says. “We constructed New York’s Brooklyn Bridge in the middle of Canada, and everything was big — even the length of the working days, which were up to 18 hours. There are more special effects than I can even think of.”

The film’s producer, Avi Arad, the man behind the twin-hit X-Men, Blade and Spider-Man series, points out, however, that special effects are never the whole story. “You can spend a fortune on computer graphics,” he says, “but if you have not hired the right actors, you are lost. Jessica Alba has the look and attitude for Susan Storm. She looks great on screen and has the ability to give all her roles a depth that is difficult to achieve from a young actress.”

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