Reese plays Fair

As we know from her huge hits, as Elle Woods in

Indeed she can. Witherspoon's Becky - a part normally reserved for the Winslets of this world - is a bitch you understand, ruthless because she's known poverty and manipulative because smart 19th-century women couldn't operate in plain sight. 'Becky Sharp is an early feminist,' argues Witherspoon. 'She is really a very modern character. She's been deprived of parents and has no place to go in the world - yet she still manages to succeed. Every success she has in her life is based on her own merit, which is a modern idea for a period story. She makes choices in the film that wouldn't be the choices I would make, but my job as an actress is not to judge that character but to find out why she is like that.' It's a revealing personal take on the character and completely in keeping with director Mira (Monsoon Wedding) Nair's interpretation of the Victorian masterpiece.

A smart businesswoman, Witherspoon knows exactly how studios tick and is willing to trade the early integrity she gained for arty hits such as Election for the power that box-office success brings. 'Mira wouldn't have been able to get the budget to make this film if it wasn't for my commercial successes,' she shrugs. 'It's a business. You can't kid yourself that you can just endlessly make artistic movies that don't make any money.' You might slot Vanity Fair in that category, having taken only $16 million Stateside - just $1 million more than Witherspoon's usual asking price (only Julia Roberts and Cameron Diaz earn more). Still, her personal notices have been lavish, and there's the inevitable Oscar talk.

Best Actress nod or not, she's pleased with her performance. 'I think the most important thing when you're interpreting a classic work is that you're not so bound by your reverence that you can't feel free enough to interpret,' she muses. 'I had a little bit of that on [Oliver

Parker's 2002 adaptation of] The Importance Of Being Earnest, where I felt so reverential of Oscar Wilde's material that it was hard for me to be free. So on this project I was determined to be able to let go a little bit and feel like I could put a little more of myself into it.'

Today America's reluctant sweetheart turns up for our interview in a maroon silk dress and her trademark blonde hair has been dyed auburn for her next role, as June Carter Cash, wife of Country & Western star Johnny Cash, in the forthcoming biopic Walk The Line. It's a return to her roots in another sense, too. Raised in Nashville, Tennessee - her father is a military surgeon, her mother a nurse - it's clear she remains a product of her old country gal/Southern belle background. We're talking 100 per cent steel magnolia: effervescent, always polite yet fiercely determined.

This, after all, is a woman who worked through her second pregnancy to finish Vanity Fair and got under the skin of Election's demonic Tracy Flick so well. As a child, Witherspoon was an overachiever, a cheerleader and debutantewho also modelled from the age of seven and landed her first movie, The Man On The Moon, eight years later. 'I got really good grades,' she notes. 'School wasn't difficult for me.'

But eventually it did get in her way. In 1996 the thriller Fear came along and she dropped out of her degree in English literature at Stanford University, brooking no argument from parents, teachers or agent. Blessed with a forceful, even argumentative, personality, she admits to being a bit of a know-it-all. 'Just ask my mother,' she groans. 'I have a lot of opinions. I get very determined for people to understand them. But I was like that ever since I was little. Sometimes I would hate that about myself.'

On the surface, she has every A-List accessory available: the Hollywood marriage (to her Cruel Intentions co-star Ryan Phillippe), two children, Ava, five, and 15-month-old Deacon, a Bel Air home and her own production company. She credits Phillippe, who she married in 1999, and the children, with keeping her grounded. 'As cool as you think you are, when you have two children smashing cupcakes into your dress before you go out for a premiere, you don't feel so cool. It's a great equaliser. Everybody can understand what it's like to be a mum.'

She makes life with Phillippe sound just as domestic and idyllic. She has no complaints if her husband crashes on the sofa with a beer to watch a game or two. 'I like the football season in America - it's fun. It keeps him busy on Sundays,' she says. Yet the couple admit to regular marriage-counselling sessions and his once-hot career has cooled as she has become not merely one of the most powerful women in Tinseltown but a role model to teenage girls.

When Witherspoon told one glossy she had cellulite, stretch marks, droopy breasts and disliked screen nudity - 'I'm prudish' - it made headline news, though you'd expect outspokenness from a direct descendant of Scottish Calvinist John Knox.