She is not a Paris Hilton-like party-hopper but one who craves her privacy. Refusing to discuss her rumoured real-life relationship with co-star Robert Pattinson, even her apparel - a grey Nike hooded sweat-top, pulled over her auburn hair - suggests this elfin actress doesn't want to be seen. Little wonder she finds interviews difficult.

'I sort of ignore this whole process,' she admits. 'It's too overwhelming. It's very intangible. Making movies - that's why I do it.'

Admittedly, making movies is what Stewart has been doing for half her life now. A Los Angeles 'native', her father worked as a television stage manager while her mother was a script supervisor. 'I grew up on a movie set, eating [the food at] craft service!' she says. 'I was an extra in movies all the time.'

She giggles at the thought of her first credited appearance - in the woeful comedy The Flintstones in Viva Rock Vegas - but admits pursuing acting professionally was an arbitrary decision.

'It was just like 'Yeah, I can do this. I can go on auditions'. And my parents were like, 'Do you realise what you're getting into?' My mum said, 'I'm not going to be a stage mum!' And unfortunately, she is!'

A year later, in 2001, Stewart was starring alongside Patricia Clarkson and Glenn Close in the credible adult drama The Safety of Objects. Then came Panic Room, David Fincher's claustrophobic home invasion story, in which Stewart played Jodie Foster's daughter. 'I was very aware of Jodie,' she says. 'I wasn't a sheltered child. I think I'd already seen Taxi Driver and Silence of the Lambs. People always told me that I looked like her too.'

Like Foster, Stewart also notched up an early appearance alongside Robert De Niro, in Hollywood satire What Just Happened?, while also impressing as a trailer park girl in Sean Penn's Into the Wild.

Having also worked with Dennis Quaid and Sharon Stone in Cold Creek Manor, is Stewart ever fearful when going up against well-established actors? 'Obviously I want to work with people that are inspiring. It's an ambitious thing, a bold, daunting thing, to work with somebody like Sean Penn. That's intimidating. Only because you want to be good.' So how was working with De Niro? 'My experience was so far from anything I expected,' she notes. 'I met him and he was so self-conscious! He could tell what a big deal it is for any young actor to meet him.'

Other young actors feel the same way about her. 'She's the most intimidating person you could meet,' says Jesse Eisenberg, who starred with her in last year's Adventureland, a comedy. 'She's just naturally cooler than you are.'

Indeed, given she has just played the leather-clad rocker Joan Jett in biopic The Runaways, Stewart does exude a too-cool-for-school aura (no doubt intensified after she was snapped by a paparazzi allegedly smoking marijuana from a clay pot). 'It was incredible playing Joan,' she says. 'Detail-wise, it was really nerve-wracking to get her. She's a really dynamic person.'

Of course, it is for Twilight that Stewart will be remembered. And, to be fair to her, she does not look down on the films. 'Typically on movies that I make, I get to follow a character for six weeks. Then it's done. In this case, I get to stay with her for four books.'

With Eclipse directed by David Slade, who previously made Hard Candy, the third instalment sees Bella forced to choose between Pattinson's vampire Edward Cullen and Taylor Lautner's rival werewolf, Jacob Black.

Currently filming Breaking Dawn, the final part of the Twilight series, I wonder if Stewart ever worries that she has attached herself to a huge franchise.

'It doesn't really bother me because in between, I get to do stuff like Welcome to the Rileys,' she says, referring to her other new film, which premiered with The Runaways at Sundance earlier this year.

Directed by Jake Scott, son of Ridley, the film stars Stewart as a New Orleans stripper who becomes a surrogate daughter of sorts to a grief-stricken James Gandolfini.

Stewart says there's no nudity involved. 'It's not a sexy movie,' she says. 'Everybody's like 'Oh my God, you play a stripper!' but it's not about the stripping. It's just that that's what she does.'

Away from film, Stewart tries to live a quiet existence in Los Angeles. As Scott says, 'Kristen is an influential figure to young women', and there's little to suggest that she's ready to implode and start falling out of fashionable nightclubs without any underwear on.

She admits she's been taken aback by the way Twilight has taken off. 'We thought it had a rather small, devoted fanbase and it would be a cult movie,' she says.

So how does she deal with the attentions of the male fans? 'They're all very polite. But the girls ... they're ravenous. I don't know what it is.' Then she remembers. 'It's Edward Cullen - that's what it is.'

The Twilight Saga: Eclipse opens on Wednesday