Panh is at the Bophana Audio Visual Research Centre, an archive he opened in Phnom Penh last year with the aim of retrieving Cambodia's memory. Next month, he'll begin directing his first feature in Cambodia, a period piece based on the novel A Stopping Against the Pacific by Marguerite Duras.

Like The Lover - another Duras novel that was adapted for the screen (starring Tony Leung Ka-fai) - the semi-autobiographical story focuses on an unhinged French family struggling to stay afloat in Indochina in the late 1940s. French actress Isabelle Huppert will star as a desperate, mentally unstable mother, while Gaspard Ulliel will play her stubbornly licentious son.

'The novel tells a part of our history,' says Panh. 'It takes place during colonial times, so I want to know: what were people like then? What did they think?'

Instead of a shoestring budget, as Panh usually has, he'll have a few million euros to spend, as well as a script by Michel Fessler, the French screenwriter behind the surprise 2005 hit March of the Penguins.

Panh looks and sounds 10 years younger than he did just a few months ago. Enthusiasm, optimism and relief have replaced the exhaustion that enveloped him for several months after he completed the documentary Paper Cannot Wrap Up Embers. To make that film, he spent two years working with a loose-knit group of sex workers at their rundown flat opposite Build Bright University in Phnom Penh.

It's not Pretty Woman. One of the women is dying of Aids. Another gets her face pummelled by an angry customer. All endure regular humiliation from their madam. They spend their days sleeping, recovering from illness and smoking yama, a form of methamphetamine named after the Hindu god of death.

Panh films it all closely - too closely for some. But for him, this isn't a style or technique so much as a kind of duty - his way of bringing the women back into a society that has excluded them. 'People don't want to be near prostitutes, like they have leprosy. They're excluded,' he says. 'I can't make a film far from people. If you film too far from people you're not with them. I'm not a voyeur. I'm not here to watch.'

The documentary has been praised for its stark and humane portrayal of sex workers, but Panh attempts to do more than that. He tries to revive them, with the intensity of his attention as well as the power a camera holds. He offers them the opportunity to create a new way of looking at themselves, and the chance to take control of their lives by giving shape to a documentary. 'They chose the time and place for shooting,' Panh says. 'We gave them time to reflect and decide what they want to talk about, or what they wanted to debate.'

One of the women is easy to confuse with an actress who has been cast in a role that's so inappropriate she recoils from it. But as the film progresses she slowly takes control of it. 'She was very strong,' Panh says. 'She had a very strong idea in her mind before shooting the film that she wanted to get out [of prostitution]. And the film became the opportunity to take the power to succeed.'

For Panh this power comes from recovering an identity that was, in his case, destroyed by genocide. After losing most of his family under the Khmer Rouge, he lived as a refugee in Paris. 'It was disturbing to see the Khmer Rouge given Cambodia's seat at the United Nations,' he says. 'I felt like I was drowning. I couldn't breath. I couldn't find a way out. I couldn't speak.' He tried painting, music and writing but 'was really bad at all of them'. Then, 'someone gave me a small camera and it felt good'.

He returned to Cambodia in 1988 and began what turned into a near 20-year effort to document his country's bewildered yet determined return from the precipice. These documentaries chart a journey that begins in the refugee camps along the Thai border and weaves its way around and finally into S-21, the Khmer Rouge torture centre. S-21: The Khmer Rouge Killing Machine debuted at film festivals in 2003 and is still going strong. In August it was hailed at the San Francisco Film Festival as 'one of the greatest documents in human history' by theatre director Peter Sellars. 'It was exhausting,' Panh says, who spent 10 years training the crew and almost three years to film. 'Most of the time we had no money, but when we had no money we filmed anyway.'

Now, the tables have turned: the surviving former leaders of the Khmer Rouge - such as Nuon Chea who was arrested last week - are facing trial for crimes against humanity.

'The trials are essential,' Panh says. 'We trust that when the trials go on if you want to know more [about] what happened before 1975 and what happened after 1979, you can. We hope people will do that, but it's very important in Cambodia now that justice can be seen.'