Fieldwork: "Enemies of the People"

Compared to the wall-to-wall dissection of the Nazi project in certain outlets, the atrocities committed by the Khmer Rouge in Cambodia in the late 1970s remain woefully under-documented. Yes, the events were brought to wider attention by the David Puttnam-produced The Killing Fields, but there's been very little in the cinema since, with the exception of Rithy Panh's sober, deliberately pedantic documentary S21 (2003), which adopted the working methods of Claude Lanzmann's seminal Holocaust document Shoah, and sought to set out the very nuts and bolts of the Cambodian genocide - the framework that made conceivable such mass slaughter.

Enemies of the People, co-directed by Rob Lemkin with Thet Sambath, follows the progress of a far more personal undertaking. Every weekend for almost a decade in the 2000s, Sambath - senior reporter for the Phnom Penh Post - left his family home in the city to travel into the country and speak with Nuon Chea, former right-hand man to Pol Pot. For Sambath - whose father died at the hands of the Khmer Rouge, obliging his mother to marry a militia who threatened to kill the rest of her family if she refused him - it's clear this is his own private battle to get closer to the truth of Pol Pot's regime.

Chea, for his part, strikes a largely unrepentant figure, speaking of how the KR were finally undermined by "treacherous activities"; when he talks of ideology on camera, it's as though he's addressing his generals. For every step forward in Sambath's progress - getting Chea to admit on tape that if the KR "had shown mercy, the nation would have been lost", thus acknowledging at least an awareness of the killings - there follows a step back. Confronted by images of Saddam Hussein's execution, an event that sparks renewed public interest in bringing the Khmer Rouge's own tyrants to justice, Chea's only lament is for "the fate of a patriot in an unjust society".

With two million Cambodians slaughtered, there had - by definition - to have been many slaughterers, a sizeable percentage of whom have thus far escaped prosecution for their actions. Whereas Nazi war criminals - many of whom had links to the aristocracy - could flee to South America as the net closed in in the years immediately following the War, the poverty-stricken footsoldiers of the Pot regime, chiefly farmers and labourers tied to the land, had nowhere else to run. It's this Sambath is relying upon when he visits some of these men in their homes, with the aim of recording their experiences to take back and show Chea as evidence of the human cost of KR policy decisions.

What we watch here is a gradual process of trust-gaining: Sambath sits down with these killers, eats with them, assures them they're unlikely to face prosecution while their superior officers remain at large, slowly working his way up and along the chain of command. He retrieves extraordinary, unforgettable detail. Two of the interviewees blithely discuss how they supped on human gall bladders in order to cool their own flesh amid the killing, literally making themselves cold-blooded; one footsoldier speaks - much as a tennis player or cricketer might of their chosen equipment - of how he came to change his grip on the knife he used after finding the alternatives uncomfortable: "I slit so many throats my hand hurt."

In places, Sambath's story threatens to get in the way of the bigger picture. There are framebreaking retreats to the journalist's video editing suite, when the material might have been better presented as filmed, and a little repetition creeps in towards the end, but otherwise this is a superior example of the documentary as collated testimony: a way of gathering together crucial oral history before its subjects can pass on - or a way for said subjects to get something off their chest and make peace with their past, perhaps easing the passage between this world and the next.

It builds to the inevitable moment where Sambath will confront Chea with the truth about his own murdered relatives, hoping for a response forged of the many hours the two men have spent in each other's company. Not to give anything away, but the outcome will depend on how you as a viewer interpret the older man's words and body language, which is perhaps why Lemkin cuts away to Sambath's camera resting atop a table shortly afterwards: what, we might wonder, was his co-director looking for? Revenge? Catharsis? A simple airing of truths, possibly: as Sambath confesses in the wake of this confrontation, "After I told [Chea] about my family, I felt better".

This weekend project has become a way of overturning decades of propaganda and lies: one of the footsoldiers Sambath interviews himself arrives at a similar conclusion. "Every time someone confesses, I feel better," he volunteers. "I want this documentary shown in the provinces, and in the cities, so the people who were killers in the regime will come forward and say 'Yes, I did that, too'." Enemies of the People leaves us with Chea being brought in from the wilderness in a police helicopter to face trial, and Sambath vowing to spend his days off from here on out among his family, starting a farm; on the killing fields of Cambodia, there are, at last, a few green shoots of hope.

Enemies of the People opens in selected cinemas today.