Gwyneth Paltrow divides her films into two categories: those she does for love and those she does for money, or "shite" as she calls them, tentatively road-testing the British vernacular. Into the former category goes Sylvia, which, she says, "because it's a movie about a poet who killed herself, [didn't] have a big audience", and The Royal Tenenbaums. Into the latter goes Shallow Hal, the Farrelly brothers film, and "this terrible movie called View From the Top that Harvey Weinstein talked me into doing". Her new film Proof, is, needless to say, a labour of love.
It's quite a robust critique, given Paltrow's reputation for limpness. After her tearful performance at the Oscars eight years ago, she consolidated her image by marrying Chris Martin, lead singer of Coldplay, and moving to north London, where she pursues an aggressively detoxified lifestyle. They have an 18-month-old daughter, Apple, and Paltrow is pregnant with their second child. (The nearest I get to names is the suggestion that she might, "some way down the road", use "Paltrow" as a middle name; her husband suggested hyphenating Apple's surname, but she vetoed Paltrow-Martin because "Apple Blythe Alison Martin is just so lovely".) She looks radiantly healthy today, like a 1970s advert for suntan lotion, and is spirited and thoughtful, not at all like her wan public image - until the last two minutes of the interview, when she delivers a lecture on abstinence that makes even the PR look surprised.
I ask if she and Martin are as dull as everyone imagines, and she winces and laughs: "There is this perception of us in this country, like, oh, they're quite boring. They do yoga and they stay home watching UK Gold." She is savvy enough to know that "boring" in this context refers to her husband's music and to her own adventures in macrobiotics. "I think we're happy. We're not looking for other things in life. We like our house. We like our kid. We like our friends." Paltrow sniffs. "I think it's sort of funny how you have to be doing coke off the ass of a stripper to be perceived as not boring these days."
In Proof, written by David Auburn and based on his 2002 play, Paltrow is suitably unconventional. She plays Catherine, a socially malfunctioning maths genius, with Jake Gyllenhaal as her fellow geek and Anthony Hopkins as geek senior: Catherine's father and a man unhinged by his own intelligence. The film uses "craziness" as a proxy for "risk-taking" (tag line: "the biggest risk in life is not taking one"), and divides the world between far-out creative folk and tragic drudges who have never made a movie nor proved a maths theorem, nor had a poetic thought, nor risen to anything more creative in life than writing lists. Lists! Pathetic!
Still, Paltrow is great in the role and a reminder that big stars can also be fine actors. She plays Catherine with the diffidence of a genuine misfit. "When I saw the play in New York, Jennifer Jason Leigh was doing it, and she's someone who can look really beautiful or really plain," she says. In defence of her own casting, she points out that the technical adviser, a real-life geek who taught them how to look natural while doing maths, "was stunning ... like a supermodel".
"Of course, it's a little far-fetched that Jake Gyllenhaal is ..." A friendless weirdo too? "Yeah. But it's a movie. Harvey Weinstein was very keen that Jake be the guy. He was not really up for the quirky thing. And he was paying, so."
Paltrow has now appeared in two versions of Proof, stage and screen, and they act as bookends to the past four years of her life. When she appeared in Auburn's play at the Donmar Warehouse in May 2002, she was, she says, having the best year she'd ever had. She was 29, living in London in a flat she'd just bought in Belgravia and her parents were over to visit. She was feeling, she says, "totally reinvigorated about being an actor and I made a pact with myself that I was never going to do anything to compromise myself again." After the run ended, she spent the summer travelling and visiting friends in Europe, ending up in Italy for her 30th birthday. "And I thought," she says, "this is the best weekend of my life."
Three days later her father, Bruce, died. Paltrow is generally perceived as the face of New York Wasp culture, but her father descended from a Russian, rabbinical dynasty. "Like, 17 generations of rabbis - you see, I really am a Jewish princess!" Bruce Paltrow was a TV director and his daughter idolised him, so much so that on reading interviews with her I've often thought that references to her mother, the actor Blythe Danner, were notable by their absence.
I ask if her relationship with her mother is more complicated than the one she had with her father. "Yes," she says. "I mean, it wasn't like Mommie Dearest, but definitely more complicated." When her father died it threw certain aspects of her relationship with her mother into relief. "Like what was fantastic and what was missing ... we've always been close but there's always been that element of friction. Me being a horrible teenager to her. Nothing really terrible or dark ... I think it's because women are more complicated than men, inherently, and relationships between them can be more complicated." Having Apple has brought them closer. "I thought, gosh, does she love me as much as I love my daughter? The my-mother-did-this, she-didn't-do-that, that adolescent stuff - it made me let go of it."
Now Paltrow is more relaxed, she says, than she has ever been; she talks about "this incredible contentment, just to understand how beautiful and simple life is". As she sees it now, she spent her 20s working too hard, never saying no to anything and succumbing to "the bad side of American psychology". When she ditched university to become an actress, she was just so amazed to find work that she didn't stop working until she was "totally burned out". She first came to England in 1995 to make the film Emma, then returned for Sliding Doors, Shakespeare in Love and Possession.
"I love the English way, which is not as capitalistic as it is in America. People don't talk about work and money; they talk about interesting things at dinner parties. I like living here because I don't tap into the bad side of American psychology, which is 'I'm not achieving enough, I'm not making enough, I'm not at the top of the pile.' It's just kind of like, I am."
The irony is that to observers, Paltrow's old life looks a lot more relaxed than her present one. She used to smoke; she used to date Brad Pitt, who, if not exactly wild, has less of the air of the vicar about him than Martin. But rumours of her iron health regime are exaggerated, she says. She doesn't diet, because the minute she tells herself she can't have something, she wants it. She says she loses weight by exercising, although it's hard to imagine why she would need to. "I like to exercise and be healthy because it makes me feel good, not because I'm going to be the next Bond girl. I think it's really important to look after yourself. But I do it for myself. I don't do it because of external pressure."
Has that always been the case? "Yeah. Except once when I was doing A Perfect Murder, I quit smoking and I had gained a few pounds. And the producer came and spoke to me about it. And I felt really humiliated. I had never been spoken to about something like that."
It's a mark of the enduring parochialism in this country that the idea of faddy Hollywood lifestyles like Paltrow's and Madonna's being lived in London causes so much fascination and delight. Paltrow gave up her flat in Belgravia when she realised that she was "the only person there under 65. I love north London; it feels much more alive." Her daughter is showing signs of an English accent already. "Her A's are very English - she says 'war-ter' and 'war-lk'. She says mummy instead of mommy. I don't mind that. I will if she starts saying 'basil' and 'pasta' the English way, which really drives me nuts."
For an actress of her depoliticised generation, Paltrow is surprisingly, and gratifyingly, strident about women in Hollywood. She hates the way intelligent actresses get shoehorned into generic parts, and thinks the only way to solve it is for more women to become directors. "Even actresses that you really admire, like Reese Witherspoon, you think, another romantic comedy? You know." She sighs. "You see her in something like Walk the Line and think, God, you're so great. And then you think, why is she doing these stupid romantic comedies? But of course it's for money and status. I just think, wouldn't it be great if all of those movies people went to see were about real women?"
Paltrow has tried to be true to this with films such as Sylvia, which she thinks is her best work. As the face of Estée Lauder, she has also peddled a fantasy version of womanhood. I don't understand, I tell her, why an actor as successful as she is would bother with a dappy advertising campaign.
"I'll tell you why," she says. "I basically stopped making money from acting in 2002. All the things I've done since then have been things I've really wanted to do, and I have not made money from them." The Estée Lauder gig pays for her art just as Calvin Klein, she says, pays for Scarlett Johansson to work with Woody Allen, "and never have to do a movie where they put her in a bikini and give her a gun". Oh, give me a break. Didn't she save any money from all that work she did in her 20s? "Good question." She laughs. "I spent it on shoes. No. Well, I had saved money, but I bought a house, and I never made Julia Roberts kind of money. I made really great money, but not the kind of money ..."
She won't tell me the most she's ever been paid for a role. I say I read somewhere it was $2m. She looks mildly perturbed. Is that offensively low? "No, not at all," she says vaguely. "That's what you read? Well, it's more than that. But..."
Paltrow intends to stay in Britain for the long term. "My husband's work is here." I wonder if she drinks more when she's in the UK than she does in the US. From the corner, the PR blurts out, "I do." Paltrow looks bemused. "I'm not really a drinker. Like the last time I got drunk was on holiday in Spain three years ago."
Wow. That's a long time sober. What's that about? "I think it's gross. I really don't like drunk women; I think it is such a bad look. I think it's very inappropriate and I don't like it."
Doesn't she feel boring at parties? "No. I think they're the idiot people and I'm the normal person. But I don't really go to parties where ... I don't really have drunk friends. My friends are kind of adult; they have a drink. But they hold their liquor. I think it's incredibly embarrassing when people are drunk. It just looks so ridiculous. I find it very degrading. I think, ooh, you're really degrading yourself right now, to be this pissed out in public." She looks startled by the strength of her own reaction and, recovering, smiles blamelessly and sweeps out of the room.
· Proof is released on February 24