Somewhere in Kilburn, North London, a Frenchman, a Spaniard, a German and I are eating sandwiches and waiting for an audience with Gwyneth Paltrow. The house, of course, is not Gywnnie’s. Oscar-winning actresses, particularly those with such over-developed privacy issues they won’t appear in public with their own husbands, would never sanction such intimacy. Instead, this is a home rented out for photo shoots and thus the owner has crammed it with a lens-grabbing mishmash of Perspex tables, tailor’s dummies, black flock wallpaper and absurd sofas, as if Salvador Dalí had styled a bordello.

The last to go in, I listen to the verdicts of my Continental colleagues with growing dismay. The German emerges patting her leather jacket pockets for her fags. “Babies,” she shrugs. “She talks about nothing else.” The writer from a Madrid glossy, who conducted his interview in Spanish, rates Paltrow’s Castilian “nine out of ten” (she spent many teenage summers as an exchange student), but sighs: “Sooo boring.” Most worryingly, the French journalist warns me to have a long list of questions: “She hasn’t very much to say.”

It doesn’t seem a good start that, when I finally enter the inner sanctum, instead of greeting me with a handshake, Paltrow offers both her elbows. Some oddball germ-aversion? No, Paltrow explains she has slightly damp hands after visiting the bathroom. In the flesh she is eerily perfect: a model’s height and frame, a teeny head with that straight fall of expertly brindled blonde hair, a look only found in nature on sun-kissed children. Her skin is poreless, her face absolutely symmetrical, the only sign that she is 35 is the sheerest crumple when she frowns between her steady blue eyes. She sits with legs curled beneath her bendy, languid body, wearing a long, soft cardigan over a cross-over ballerina top, yoga-ish trousers, strapped ballet flats and, on her wedding finger, a major rock. She looks, in fact, airbrushed, computer-generated, more avatar than real woman.

Which probably explains her casting in Iron Man, a movie based on the Marvel comic, in which she has a minor part as the hero’s assistant, Virginia “Pepper” Potts. The real question is why an actress, once dubbed the first lady of Miramax, star of classy but accessible movies such as Emma and The Talented Mr Ripley, besides more “difficult” indie flicks such as Sylvia (in which she played Plath) and Proof (a tortured maths genius), an actress prone to banging on about her “body of work”, should choose this multiplex-pleaser, described by its star, Robert Downey Jnr, as “the ultimate nerdgasm”.

Of course, it could be money. Iron Man, with its computer game and mega-merchandising, will have landed her a mighty pay cheque. But the reason appears to be even simpler: Paltrow has, at this moment in her life, little interest in being a movie star, her ranking in the Hollywood pantheon or what anyone beyond her immediate family thinks about her. She chose Iron Man because it was being shot 15 minutes from her house.

“When I had my daughter, I fell so madly in love with her that the thought of being away from her for even a day made me sick,” she says in her rather bored monotone. Her dippy locution – many “likes” and “totally amazings” – which is born of living her first 11 years in LA, combines with the nasal drawl of her teenage years in Manhattan. “It’s probably quite obsessive. But I couldn’t bear the idea. Also I had worked such a lot in my twenties, I think I had burn-out a bit. Then I had this amazing new thing come into my life and I just had no interest. It was, like, the fire was out. I had no desire. And then I basically didn’t work. I did Proof when I was pregnant with Apple and stopped when I was about four months and then I just cooked and did yoga...”

In the run-up to turning 30, Paltrow made 21 films in nine years, living the necessarily self-promoting Hollywood life, complete with tabloid-fodder failed relationships to Brad Pitt and Ben Affleck. She was a diligent red-carpet attendee and fashion show front-rower, her impeccable glamour and poise winning her praise as a throwback to old Hollywood, in particular Grace Kelly, whose cool yet strong-jawed looks she shares, along with a beautiful blankness that allows directors to project their specific vision upon her.

But it was a life of 5am calls, hair and make-up trailers, 14-hour working days, pressure and tumult. And Paltrow says, since the birth of Apple, four, and Moses, two, she has just not been able to figure out how to square this old life – one she had wearied of anyway – with motherhood. “It was very hard for me to get my head around the idea of how one is supposed to do this. I mean, I’m a woman and a mother, but also an actor. And how will I not feel terrible if I go to work? So I said I’ll wait until Moses is a year.”

Apart from a minor role in her brother Jake’s directorial debut, The Good Night, and a few cameos, she has barely acted for three years (although she has earned gazillions as head spokesmodel for Estée Lauder). “So my agent got this call about a movie,” she gushes. “It’s Iron Man and it’s a comic and I said, ‘Okaaaay…’ and then he told me who was in it. And I’d always wanted to work with Robert. Oh, my God, he is sooo great! You have no idea! It’s insane! And Jeff Bridges! Oh, I love him so much! I thought I can’t be prejudiced because it’s a big-budget movie with a dream cast. Like, if it was a tiny movie, I’d be doing it. Then Robert called me and said, ‘Don’t you want to be in a movie that people might actually see?’ I thought, [she giggles] ‘What would that be like?’”

And then it turned out the studio was close to her home in LA. “So our nanny, who we love, would bring the kids for lunch, and I didn’t work five days a week. And a lot of time I would work from noon to midnight so I could be there all morning. But that is not the norm.”

Paltrow’s bodyguard-driver enters bearing a cup of herbal tea – “Oh, thank you, Terry!” – and removes a similar empty one.

It sounds irritatingly precious, the notion that a powerful, rich movie star married to an equally wealthy rock star – Chris Martin, lead singer of Coldplay – might have trouble figuring out her childcare. But the point is Paltrow doesn’t need to work, and, with winning an Oscar at age 26 (for Shakespeare in Love), has nothing left to prove. But more than that, motherhood, after her turbulent A-list decade, restored the stability and wholeness she herself enjoyed as a child.

Explore Film

• Film Reviews

• Trailers

• DVD

• London Film Festival

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010