It's easy these days to write off young actresses as being manufactured products of Hollywood.

Yet if they take a page out of the book of the likes of Angelina Jolie or Jessica Alba, they might realise that there's a lot more to navigating Hollywood than just looking pretty.

That's not to say Jolie and Alba aren't two of Hollywood's most beautiful actresses. They're just two of the smartest - and two of the fittest.

While Jolie has demonstrated her physical prowess in Tomb Raider, Wanted and Salt, Alba, who started out in television's Dark Angel and who was the sole female superhero in Fantastic Four and its sequel Rise of the Silver Surfer, is now kicking some serious ass in the Mexploitation movie, Machete, by her Sin City director, Robert Rodriguez.

In fact, now happily married to film producer Cash Warren, the 29-year-old mother of Honor, 2, is proud to have played a Mexican action heroine in her first Latina role.

"I have always been attracted to strong, powerful women, like in Aliens, Terminator, Nikita and even The Professional, Natalie Portman's character," says Alba, who was born in California to a Mexican-American father and a mother who is of French and Danish descent.

"I think we need more strong women in movies. That is kind of why I got into the business. I also think that women of colour should be leads. I think we can break down the stereotypes where you have to have blond hair and blue eyes and that is the only thing that is acceptable to be a leading lady, and smile all the time and be a victim and be saved. I just love being able to hold my own in an action film."

Alba is sitting beside the film's remarkable 66-year old star, Danny Trejo, whose chest resembles a brick wall and whose face looks as if he has run into one.

Despite their age difference their characters fall for each other in the movie and even if this might seem incredible, today they really share a certain charm. Though when Trejo says something contentious, Alba has no qualms about whacking her co-star, a one-time San Quentin inmate, in the chest, which only makes Trejo grin in delight. It's quite a pairing.

Though the Machete character had been gestating since Rodriguez first met Trejo on the set of Desperado, the new movie began as one of the fake trailers in the Tarantino/Rodriguez double feature Grindhouse.

Audiences loved it so much that Rodriguez turned it into a full-length feature, with Trejo in his role as the Mexican lawman turned legendary outlaw. Alba plays special agent Sartana Rivera, the right-wing immigration officer sent to capture him, who comes to see his point of view - and clearly more.

Action movies are cathartic for Alba, who maintains she is shy.

"The last time I hit someone was a long time ago. I think I was 11 and I beat this kid up. This boy, he was being disrespectful and I got even and then was sent home. I was very much a tomboy.

"My dad had wanted a boy, I was his first and he was 19. He would never say he was bummed out about it but I think he may have been, so he taught me how to throw like a boy. I always played sports with the boys, I climbed trees and I am very competitive."

So she never had Barbie dolls? "I had Barbie dolls but they would jump off buildings and so on!"

The comic book nature of Machete came as some relief to Alba who had previously been the victim of Casey Affleck's grisly serial killer in The Killer Inside Me.

"It was a challenge," she says. "I'd never done a role like that. She wanted love from this man when he was being violent with her. That's what she liked. So it was very dark and tough for me and obviously far from me as a person."

Directed by Michael Winterbottom and based on Jim Thompson's 50's pulp fiction novel, The Killer Inside Me represented a conscious desire by Alba to instigate a change in her career.

"After I had my daughter I realised that for most of the previous decade I'd done what my agents wanted me to do. I'd made a lot of commercial films and it was about the budget and about whether the film was going to be released globally or not. It wasn't about the film-maker and I just lost my passion completely. So when I took time off I asked myself, 'What do I really want?'

"Now, to spend time away from the most precious thing in the world, which is my daughter, I want it to be worthwhile. So I only want to work with great film-makers and I don't care if it is a big movie or a small movie. I just want to be inspired and I want to evolve."

Will people perceive her differently for jobs after her making such a radical choice? "Probably, but I am much more comfortable in my own skin and I feel like a grown up. Before I felt like a girl."

Who: Jessica Alba
What and when: The Killer Inside Me (opens at cinemas November 4), Machete (November 11)