'I'm not an easy girlfriend

After Tessa is brutally raped and murdered, Fiennes's character sets out to find out why, uncovering a conspiracy by pharmaceutical companies to test their unstable new TB drugs on unsuspecting Africans in the continent's largest slum, Kibera, just outside Nairobi.

Weisz is wonderful in the role; both she and Fiennes, I am sure, will be Oscar contenders. I ask what Fiennes was like to work with. "It is hard to describe someone in just a few words," she says. "I love to improvise and so does he. It was like a jam session. Some of the best lines were like jazz coming out of his mouth."

The decision to film in Kenya caused huge problems for the film's producer, Simon Channing-Williams. The country was initially considered too dangerous, but Channing-Williams managed to enlist the support of many Kenyan government ministers - and Britain's High Commissioner in Kenya, Sir Edward Clay.

During the two months of filming, Weisz stayed with a close friend, a jewellery designer called Carolyn who grew up with Masai tribespeople and lives on the edge of the Rift Valley. Weisz would fly there each evening in a two-seater plane after a 14-hour day on set.

"Filming in Africa was definitely life-changing," she says. "I did lots of research with Oxfam before I went there, and they let me meet people who had worked in the field and hear their stories. But the real inspiration came from a female activist who let me join her while she was counselling women in the field. She is HIV-positive and would talk to the women about how to make themselves safe."

There is no sanitation, no running water, no electricity in Kibera. "It's home to 800,000 people, a place where Aids has made orphans of thousands of children," says Weisz. "It's a no-go area. White people don't go there, even the 70 Kenyans working on the crew had never been there. The director said, 'Brazil is poor, but it is like Switzerland compared to Africa.'"

Weisz has spoken in the past about carrying around a lot of guilt. Everything has, after all, turned out remarkably well for her. "Look," she says, banging the coffee table between us with her hand. "My guilt doesn't help anybody. Feeling guilty is patronising. Even though the people there are poor, they are incredibly generous.

"The crew was deeply affected by what we saw and, in exchange for filming, we built a school, showers and lavatories, installed three 10,000-litre water tanks, built a bridge across an open sewer, and set up a charity called The Constant Gardener Trust to build a secondary school in the north, which is where the second half of the film takes place. At the moment, girls only stay in education until they are 12 and are then married off, so our schools will keep them in education longer."

At another secondary school the production team paid for the all-girl choir (all Aids orphans) to travel to a national contest. "The girls finished second," Weisz says proudly. She insists it was Channing-Williams, now somewhat of a hero in Kibera, who made sure "there was something real that we could do.

He made sure that, of the 58 speaking parts, 34 were played by Kenyans, and ensured they were paid £5.60 a day when the average wage is less than £1 a week. He even persuaded Kenya's minister for roads to travel with him to show him how appalling the slums were. It was a very small amount of money for us. I will never forget those children."

She says she is definitely going to go back to Kenya, maybe to hold a gala premiere in Nairobi to make sure all the rich people dig deep into their pockets.

Is there anything that keeps her awake at night? Growing older, maybe? She looks remarkable in The Constant Gardener, despite not wearing a scrap of make-up. " Growing older doesn't worry me, not at all. I hope I will still be getting jobs when I am 70. I worry that we are destroying the planet."

I ask whether it is true that Aronofsky proposed to her on one knee in Times Square. She snorts. "No, that is not true. Although we are engaged." They share an apartment in Manhattan and have just made a film together, The Fountain, which stars Weisz and Hugh Jackman.

How was that, working together? "It is lovely to be with someone who is good at what they do," is all she will say. I do manage to wrestle out of her that her close friend Narciso Rodriguez will be designing her wedding dress, and that they will be getting married in New York's oldest synagogue. I also discover that she likes shoes, has a tattoo and that "I definitely want to have children".

I don't even bother talking about her former boyfriend, Sam Mendes, and whether or not, if she was still with him, she would be as famous as Kate Winslet. It is quite obvious that she would detest being as famous as Kate Winslet. She does let slip, though, that she is "not an easy girlfriend". Why does that not surprise me?

"I'm hard on myself," she says finally. "I have flaws like any person. None of us is cool in real life.

Growing up in those slums, that is real. That is hard. Being me isn't difficult at all."

The Constant Gardener is being shown at Venice Film Festival on Friday, 9 September in competition and is released in the UK on 11 November. To make a donation, visit www.constantgardener trust.org