We're meeting in a suite at the Dorchester. Weisz plays down the significance of the outfit seen around the world - "He's really good," she murmurs of Mouret, "he's really got something" - but she's not moved to say much more.
She's lived in New York for four years now, with her fiancé Darren Aronofsky, the acclaimed - and hip - director of the low-budget mathematical thriller Pi and the sex/drugs/ amputation shocker Requiem for a Dream. But for all this starry A-list status, Weisz isn't much of clothes fiend.
Of much more interest was the chance to come home - and to support the film in which she stars. "It's exciting to be at the opening of the London Film Festival," she beams, a picture of relaxed elegance in a little black dress.
Weisz, from north London, is the eldest daughter of a Hungarian medical-inventor father and an Austrian psychotherapist mother. "This is my home town," she says, "so I was extra nervous and excited last night. It felt special that I should grow up to be a girl in a film that was opening the LFF. OK, it's directed by a Brazilian, but it's quite British, I would say."
In The Constant Gardener, directed by Fernando Meirelles (City of God) from the John le Carré novel, Weisz comes of age as an actor. In this politically charged thriller about the nefarious activities of pharmaceutical companies in Africa, she plays Tessa Quayle, the new wife of the British diplomat Justin Quayle (Ralph Fiennes), and an idealistic young activist.
Weisz's performance is electrifying. This is only partly to do with the fact that we know from very early that Tessa has been murdered (the narrative is told partly in flashback, flitting between London, Kenya and Berlin). As he proved in City of God, the quasi-documentary depiction of perilous life in the favelas of Rio de Janeiro, Meirelles can handle tricksy and multilayered tales.
Weisz was so keen to bag the role that she dropped everything and flew to London from Los Angeles for the auditions (and flew back, all within 24 hours). She went into her meeting with Meirelles, she has admitted, like a "hurricane of passion".
Why so keen? "It was everything," she shrugs, as if to say it was a no-brainer. "It was the character, a brilliant, once-in-a-lifetime role. It was the story. It was Ralph Fiennes. And it was Fernando - I'd seen City of God. That surpassed my wildest fantasies about what the film could be."
In the film, Justin Quayle is the epitome of Establishment disengagement. While he would rather tend his beloved plants (hence the film's title) than stick his neck out, Tessa throws herself into the desperate, day-to-day struggles of Kenya's teeming poor.
After she and a local doctor begin investigating the drugs supplied to - and tested on - the locals by Western companies, their fate is sealed. When Justin sets out to find the truth of what happened to his wife, he has to face the possibility that she was not all she seemed. In The Constant Gardener nothing is clear, and everything is all the more credible for it.
"Absolutely," Weisz nods. "I'm one of the 'good people'. But I'm not an angel. She's an infuriating person. That's what I love. It's a woman who's very flawed and is a human being like the rest of us - but who happens to be very driven to unearth these injustices. And where are the bad guys? It's very hard to say. Is it government? The corporations?" She praises Meirelles for pulling all this together. "He's a unique master-storyteller. Film is all about the director. It really is. I've been a bit slow to realise that."
Well, perhaps. It seems that this Cambridge graduate, now 34, has been engaged in a dizzying search for collaborators and stories that suit her best, ever since her appearance in Bernardo Bertolucci's Stealing Beauty in 1996.
At the start of her career, she toyed with the idea of changing her name to Vyce (that's how it is pronounced). Her agent witheringly said it "looked like a number plate". For her first screen role, a bit part in a schlocky American detective series shot in Israel, she was Kenya Campbell. That was partly a tribute to an early mentor, the actor and playwright Ken Campbell, whose one-man show Pigspurt "blew my mind," she says. "I used to meet him in his office, which was a bench in the Thames Valley. He gave me lessons in how to tell stories." And the name change also happened, she confesses, partly through mortification at having to play a cat burglar who dies in the arms of the hero, croaking: "I love you." But she was just out of university, and it paid £3,000. "It was an awful lot of money. I'd never had a paid acting job."
Those came thick and fast. Roles in interesting low-budget British films such as Beautiful Creatures and Michael Winterbottom's I Want You were overshadowed by parts in big popcorn numbers The Mummy and The Mummy Returns, and lame Hollywood fare such as Confidence and Runaway Jury.
Her looks elevate her performances, yet have trapped her. She was brilliant as the love interest who finally gives Hugh Grant's moneyed loafer a kick up the rear in About a Boy, and compelling as the conniving beauty in the Almeida Theatre's version of Neil LaBute's The Shape of Things (but seemed less confident in the film version).
In this year's special-effects-heavy comic-book adaptation Constantine, with Keanu Reeves, she was more eye-candy than anything else. Making this film was "fun in that when you get to see the movie, you go, 'Oh wow, that's what the animators did, they drew that?'" She'd like to say it's challenging acting against blue- and green-screen, "but it's not. It's like being a kid - just imagine a scary monster. I'd say it was incredibly easy."
Weisz is refreshingly honest about the roundabout route of her career. "You have sometimes to go into the unknown. You just think, 'That sounds interesting.' It's about having different experiences. And the more you do, the more you know what you don't want to do."
She may have learnt a lot about patience and taking the long view from Aronofsky, who has been working on his next film, The Fountain, for six years. "He's a bit like Tessa - he'll just keep on going," she says, with affection. Aronofsky wrote The Fountain's century-spanning fantasy script for Brad Pitt. Cate Blanchett was attached, and the budget was a thumping $100m. Then Pitt pulled out, and so did Blanchett. "So Darren downsized the film. Rewrote the script. Took out some of the big battle scenes, so it became a $30m movie. And [studios] still said no. Because it's still a lot of money for something they might think is just too out there. But I don't think it is."
She would say that, you might think, not least because she's in it. The male lead is now Hugh Jackman, and it's in post-production. "It looks big, but it's a $30m movie shot to look like a $100m movie."
Away from the excitement that has greeted The Constant Gardener, a huge hit in America, Weisz has family and friends to see while she's back in the UK. Among the latter is Susan Lynch, her Beautiful Creatures co-star, with whom she plans to film a script they've written about a pair of female "entrapment officers". She's developing two more scripts, both comedies, in which she'll star. One, in America, is an original idea; the other is based on the autobiography of a female journalist. "Not Julie Burchill! I'm a big fan of Julie, though. My mum sends me her articles with marginalia - 'I so agree!'"
Weisz will be back in the UK next spring, filming Death Defying Acts, an Edinburgh-set drama about the friendly rivalry between Harry Houdini and Arthur Conan Doyle. Guy Pearce plays Houdini. The life of the illusionist is a long-standing interest of Weisz's, partly motivated by the fanciful thought that he was a distant relative - he was born in Hungary as Erich Weisz.
What of the rumours that, having previously fronted a Revlon campaign, she's to be the new post-Kate Moss face of Burberry and has been shooting ads with the photographer Mario Testino and the actor Ioan Gruffudd. "I can't comment on it! I'm so sorry! I'm not allowed to say anything, it's awful." We'll take that as a yes, then.
Not that Weisz needs the approbation of the fashion world, lovely dress or not. After almost a decade, with The Constant Gardener she has become an actor to be reckoned with. "I think I've just been experimenting. I've not been that strategic. Maybe I've been lost in places? I don't know."
She grins. "I feel that with this film I'm getting close - maybe I won't get any closer - to the kind of film I'd like be in. It's very rare, this kind of film. They don't come along often. But it's definitely the film I'm most proud of. Definitely."
'The Constant Gardener' opens todayReuse content