Like most of the world, Robert Redford learnt most of what he knew about one of the most gruesome episodes in modern history from the Oscar-winning film The Killing Fields.

At least that was the case until the Hollywood legend met Oxford-based film maker Rob Lemkin. The two men met in Utah at the Sundance Film Festival, founded by Redford, where Mr Lemkin’s film was to win a major award, securing his reputation across the globe as a documentary film-maker to watch.

His achievement was remarkable, not so much as a result of the film’s limited budget and the three years’ hard work put into it, but because of its content, shocking even in an age where atrocities and mass killings are almost commonplace.

For Enemies of the People provides first-hand accounts of the genocide inflicted upon the people of Cambodia by the Khmer Rouge, one of the 20th century’s most brutal regimes.

It is little wonder that Robert Redford readily recalled Roland Joffe’s 1984 Oscar-winning film The Killing Fields when he met the Oxford director. That powerful drama about the desperate days of the Khmer Rouge, seen through the eyes of a New York Times journalist and his Cambodian interpreter, left indelible images of what happened under the rule of Pol Pot, when two million people perished.

Mr Lemkin said: “He took me out for lunch and said he knew the story of Cambodia from the Hollywood film. But he didn’t know the real story. My own personal connection with Cambodia is non-existent. But my connection with genocide is not; many of my father’s family died at the hands of the Nazis and a rather remote relative, Raphael Lemkin, even coined the term ‘genocide’.”

Redford certainly knew far more about the real killing fields after seeing the Lemkin film, which picked up the World Cinema Special Jury Prize for Documentary at the Sundance Festival.

Instead of movie stars, this film features men and women who perpetrated the massacres, from the foot soldiers who slit throats to the party’s ideological leader, Nuon Chea, Pol Pot’s right hand man, the infamous Brother Number Two, who broke a 30-year silence to give his testimony, never before heard or seen.

Imagine if Heinrich Himmler had not taken his own life but having survived the war and Nuremberg Trials sat down for hours to discuss on camera the scale and motivation for Nazi crimes against humanity: for millions of Cambodians, Mr Lemkin has achieved something almost akin to that.

But Enemies of the People, which will be shown in Oxford next month, like The Killing Fields, is also about a remarkable journalist, who co-directs the film.

Thet Sambath, who had lost his own family in the killing fields, is regarded as Cambodia’s best investigative reporter. The film is also the story of this remarkable man’s own journey to discover not how, but why his family had died.

Mr Lemkin, a father of four, who set up his own film company, Old Street Films in Hurst Street, East Oxford, 20 years ago, has long been interested in Asian politics.

His wife’s family is Burmese and in recent years his television work has focused heavily on Asian history. One of his best known documentaries, Who Really Killed Aung San?, shown on BBC 2, controversially re-examined the assassination of the founder of Burma.

But his fascination with Pol Pot, who died in 1998, was stirred ten years ago when he was making a BBC documentary about a mysterious Malaysian revolutionary called Chin Peng.

“He came to London for the premiere and in the back of a taxi he told me that he had stayed with Pol Pot.”

Rather than appearing an all-powerful dictator, Peng described Pol Pot as being “like a rabbit in the headlights”, way out of his depth after seizing power.

With the long-awaited UN-backed genocide trial of some of the murderous Khmer Rouge figures under way, Mr Lemkin flew to Cambodia in 2006 with the idea of making a film about the trials.

“I quickly realised the person I needed to speak to was Brother Number Two. He lived on the Thai border and I drove a day up to see him.”

But the old man insisted that he knew and had seen nothing.

Mr Lemkin, however, then met the great journalist, who was having greater success.

Having lost both his parents and a brother in Khmer Rouge’s Killing Fields, Sambath had already spent years in search of those who had orchestrated the genocide, spending hundreds of hours, to the detriment of his family life, conducting interviews to win the trust of mass murderers.

Like Mr Lemkin, he, too, had initially found the most senior surviving leader a hard nut to crack — but patience had begun to deliver the richest of rewards.

Sambath recalled: “Working with Nuon Chea was not easy. He is secretive and would not say anything different from what he told Western journalists — ‘I was low ranking. I knew nothing. I am not a killer’.

“I knew from the beginning that working with him would require a lot of time — years, not months. Then one day he said, ‘Sambath, I trust you. You are the person I would like to tell my story to. Ask me what you want to know.’ For the next five years he told me the truth, as he saw it, including all the details of the killings.”

When Mr Lemkin suggested turning Sambath’s material into a film, the journalist was even able to persuade Nuon Chea to allow the English documentary maker to film their detailed interviews.

Over three years, Mr Lemkin realised that he was filming one of history’s most remarkable confessions.

“Brother Number Two saw something in this young man: he saw an unparalleled opportunity to give his own account of his time in politics. He spoke in detail for the first time about how he and Pol Pot, the two supreme powers in the Khmer Rouge state, decided to kill party members whom they considered enemies of the people.

“Nuon Chea and others in the film are real people who carried out unimaginably horrific things. The film is about what it is like being these people now and what it was like for Sambath to be with them. It is the story of one man’s journey to the heart of the killing fields.”

Sambath’s father was killed by the Khmer Rouge in 1974 when he refused to give them his buffalo. His mother was forced to marry a Khmer Rouge militiaman and died in childbirth in 1976.

Mr Lemkin said: “There have already been many films and books about this period. But our film is a new way of telling the story of this event, one of the biggest mass killings in recent times.”

The film’s two directors also uncovered a whole network of collaborators who made remarkable confessions about what they did. Simple peasants, who were forced to execute their countrymen or risk execution themselves, gave personal accounts of mass slaughter. In one chilling scene Sambath asks one of them to demonstrate on a relative with a plastic knife how he slit his victims’ throats from behind.

The two men believe seeking to get into the head of mass killers can be justified.

“Only a documentary film allows you to get close to the characters in a layered way. There are different levels that you can relate to these people. Sometimes you hate them, sometimes you’re fearful of them and sometimes you pity them.

“For the first time we see how orders created on an abstract political level translate into foul murder in the rice fields and forests of the Cambodian plain.”

Sambath said: “Some may say no good can come from talking to killers and dwelling on past horror. But I say these people have sacrificed a lot to tell the truth. In daring to confess, they have done good — perhaps the only good thing left.”

They also believe that the film, which has caused a sensation in Cambodia, will help bring about reconciliation, allowing the country to move forward.

Mr Lemkin, 48, said: “I think it will make a massive contribution to the healing of the country. As for Sambath, I see him as a man trying to make sense of a nightmare of his childhood.

“When he finally understands the genocide, as he says he does, he is achieving inner peace by being able to situate his personal loss in the wider sweep of history.”

Nuon Chea, now 83, has since been arrested by the United Nations and faces charges of war crimes, crimes against humanity and genocide.

It looks likely that the film and some of the 160 hours of taped interviews, will be sought by the prosecution.

“We are now finding ourselves in a tough situation,” said Mr Lemkin.

“I know Sambath would be reluctant to hand over the tapes on the basis that people interviewed for the film were offering their accounts for the historical record, not to incriminate themselves.”

He recalls sitting in the Radcliffe Camera, working his way through pages of interview transcripts.

“I would be surrounded by Oxford University students working for their finals. And there I would be reading through these horrific confessions.”

But the effort proved worth it, with the film collecting awards and standing ovations at festivals from Finland to California.

But what about the big one? Could it follow The Killing Fields as an Oscar winner?

“Well, we have been approached by people from the Academy.”

If it came to a close fight he already has the Sundance Kid on his side — as well as millions of Cambodians, who will view Enemies of the People as a life-changing experience rather than a film.

  • Enemies of the People is being shown at the Ultimate Picture Palace, Jeune Street, Oxford, on Saturday, April 24, at 7.30pm.