The secret of the Dardenne brothers' Palme d'Or success

The Cannes Film Festival, in 1999. Tucked away in that year's competition was a film that didn't get a star-studded evening gala. Rosetta was unveiled in the graveyard slot, at 4pm on the last Saturday afternoon, when many people had long since left town. The programme described it as the tale of a young woman living in a trailer with her alcoholic mother, struggling to hold on to her dead-end job at a waffle stand. So, it came as quite a shock when the film's directors, the Belgian brothers Jean-Pierre and Luc Dardenne, were named as the winners of the Palme d'Or, and their young star Emily Duquenne got Best Actress.

And last year, at Cannes, with their latest film, The Child, the Dardennes joined that very small, select band of directors who have won the film festival's most prestigious award, the Palme d'Or, twice.

All of their films are set in the grey, run-down steel town of Seraing in eastern Belgium, where they grew up. Made on tiny budgets, their work hovers on the margins of mainstream society, in the world of the have-nots. But, far from looking drab, their cinema is exciting and full of suspense: an urgent, fast-moving camera races after the characters, stalking them through their complicated lives and sweeping the viewer along in a process of total immersion.

And this is no agitprop. After Rosetta, legislation was passed in Belgium protecting the rights of young, low-paid workers, but "it was pure chance", Jean-Pierre insists. "There was already a bill going through, and the minister took advantage of our award to call it the Rosetta Law. But we never intended to get laws changed." Luc adds: "Of course, we always hope our films will speak to people, disturb them, but we never hoped to change the world."

The Child centres on Sonia, a young woman who has just had a baby, and her boyfriend Bruno, a petty criminal living on the streets. Bruno is carefree (some would say, shamefully irresponsible) and money burns a hole in his pocket. Short of cash, he impulsively sells his son to child-traffickers, and is puzzled when this sends Sonia into a state of shock. After all, he points out, they can always make another one.

The film began, Jean-Pierre explains, with their glimpsing a young woman pushing a pram down the street in a weird, manic manner. "We saw her several days in a row, and wondered about her. Then, a year later, when we started work on our next film, we suddenly started talking about her again, I don't know why. We thought it would be interesting to devise a story about her, and somehow it turned into the story of a boy who can't accept fatherhood."

Animated and genial, Luc, 51, and Jean-Pierre, 54, give interviews with good grace, but they make no secret about being uncomfortable at their red-carpet status. And as with other fraternal film-making duos - the Coen or the Quay brothers - the interviewer can feel slightly intimidated, simply by virtue of being outnumbered. This feeling is undoubtedly shared by their actors. Jérémie Renier, who plays Bruno, made his screen debut at 14 in the Dardennes' 1996 film The Promise, as a youth helping his father smuggle illegal immigrants into Belgium. Since then - like many of the brothers' other discoveries - he has developed a successful acting career, but he admits to being daunted by the prospect of collaborating with the Dardennes again. "We told him he'd be working under the same conditions as everyone else - no en-suite bathroom, no girlfriend, no driver, no pets," Luc says, pounding the table. "That's important, in order to create the right atmosphere."

Déborah François, the elfin 18-year-old who plays Sonia, the young mother, was recruited by the brothers' usual method: radio and newspaper ads inviting local girls to send in their résumés. She was one of 200 to be auditioned. After three goes, she landed the role, but, "they never explain anything about the character's psychology," she says. "You mustn't ask them about that. So you never know if they're happy with what you're doing. But when they're not satisfied, they tell you. They say, 'You must feel things in your belly'."

'The Child' is out now