THEY put on their best showbusiness smiles for the cameras last night, but Rachel Weisz and Ralph Fiennes knew that behind the glamour of the premiere of The Constant Gardener they were helping a greater cause.

The actors, who were in Leicester Square for the opening film of The Times bfi London Film Festival, have become patrons of a charitable trust to give something back to the Kenyan community that hosted them during filming.

John le Carré, on whose book the film was based, is also a patron of The Constant Gardener Trust (www.constantgardenertrust.org ), which will help children in Kibera, the largest slum in sub-Saharan Africa.

Mr le Carré said last night: “The film-makers did me proud. They did themselves proud. It’s not the film of the book, it’s the film of the film. You come out feeling the same stuff as you do when you come out of the book.”

The film - the only big-screen adaptation with which le Carré is truly happy - stars Fiennes and Weisz as a diplomat and his wife who become embroiled in the deadly politics of pharmaceutical companies operating in Africa.

The author said that his book and the film are just “a holiday postcard” compared with the real exploitation taking place. He and the film’s two leading actors have made large donations and have pledged to help with fundraising for projects, including building a secondary school and medical facilities.

Fernando Meirelles, the director, whose last film City of God was shot in the violent shanty towns of Rio de Janeiro, may also become a patron. Simon Channing Williams, the film’s producer and a trustee of the charity, said: “It was an extraordinary experience for us all.

“There are a number of people, including myself, who would agree that filming out there has very nearly been a life-changing experience.”

Mr Channing Williams said a decision had been taken early on to use as many local crew as possible, to pay them well over the going rate and to invest in schools, medical care and infrastructure in the communities where they were working. Now the emphasis is on developing the trust to work in deprived areas across Kenya, he said. “The film industry has a reputation for taking a slash- and-burn approach, and I’m afraid that’s well deserved,” Mr Channing Williams said. “Typically you go in, film and get out. There are a lot of people in the industry who care very deeply about how they leave a location but there are others who are less careful.”

The violation of Ko Phi Phi, an island in Thailand, during the making of The Beach is a recent example.

Some location excesses are the stuff of legend: filming on The African Queen in the Belgian Congo was frequently put on hold because John Huston, the director, was obsessed with shooting a bull elephant.

More recently David Lean is rumoured to have enlarged the Marabar caves in India with explosives to shoot a scene in A Passage to India.

Werner Herzog, the German director, once flooded a small Dutch town with 10,000 rats while filming Nosferatu. When the crew moved on, having fallen out with the local community, they left the rats behind.

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