After taking the world by storm with The Act of Killing, his surreal and disturbing 2012 documentary about these events, director Joshua Oppenheimer has now made a companion piece, or perhaps prequel, The Look of Silence, about the same subject matter. Adi Rukun confronts Amir Siahaan in The Look of Silence. In this film he focuses closely on the inhabitants – both murderers and their victims – of one village in North Sumatra. Some were armed with machetes and the certainty of religious righteousness and ordered by the army to hack their neighbours to death and discard them into Snake River. The victims were demonised as atheists and communists, and still are. Drinking their blood was supposed to protect the killers from the psychological consequences of their crimes. "Human blood tastes salty and sweet at the same time," reminisces one, old and toothless, but still evidently satisfied with his role. "Two glasses per night; that's enough," says another.

At the moral heart of the new film is Adi Rukun, a 40-year-old optometrist born shortly after his brother Ramli became a victim of one of those murderous nights. Adi Rukun's brother was killed in the Indonesian anti-communist purge. Ramli's death, it's revealed as the film progresses, was even more gruesome than most. His father, Rukun, was so traumatised by it that afterwards he lost one tooth everyweek until he had none left in his mouth. But the boys' mother, Rohani, unlike most people in these tiny villages, could not force herself to pretend it never happened. She regarded her youngest son Adi, born after these events, as a kind of replacement Ramli and spoke to him regularly of the circumstances of his brother's death. Adi was a surprise guest at a screening this week of the new film to a small audience in Jakarta. He said that, when Oppenheimer came to the village in about 2005 to interview his mother, Adi had volunteered to visit the killers as an optometrist in the guise of checking their vision. Rohani, mother of Adi and Ramli, in The Look of Silence.

As the test lenses pop in front of the rheumy eyes of these superannuated monsters, Oppenheimer's camera observing them in aching close-up, Adi asks: "Is that more clear? Less clear?". It's a powerful metaphor. As he tests their vision he's also gently probing their memories, seeking clarity for himself about their mutual history. Their answers, for the most part, demonstrate pride and boastfulness; an eager retelling of gory details. Inong Sungai Ular has his eyes tested in The Look of Silence. There is a good deal of swagger here. Almost 50 years have passed, and those who wielded the machetes have got away with it. They've lived long lives and enjoyed wealth and power (often built on the back of their crimes). Indonesia has never faced its past or apologised to the victims. "The perpetrators are still in power all across the country, as we all can see," Adi tells me after the film has finished rolling. The stigma attaches not to the killers, but to the victims and their families, who are still tarred with the communist brush.

Rukun was so traumatised by his son's murder that afterwards he lost one tooth per week until he had none left. Inevitably, at some point during Adi's optometry appointments, though, he confronts them with the fact that he is a victim of their crimes, that his brother was murdered in the purge. The killers respond with offence and the full gamut of denial: "We were acting under orders"; "The communists were Godless animals"; "Let the past rest, don't dig it up". And, from a former militia commander turned long-time politician: "If you had come to visit me under the Suharto dictatorship, you can't imagine what would have happened [to you]". These awkward confrontations do not normally happen in Indonesia, Adi says after the film: "It's truly covered up. We pretend not to know … Nobody dares talk about it." The consequence of such silence is predictable. A scene in a schoolroom shows children being told of the evil of the communists and how the government was right to take strong action. The collective impulse to forget is strong, but the truth is available to anyone who encourages these old men to boast. In sequences shot by Oppenheimer more than a decade ago, groups of old militia men giggle as they reminisce about their exploits at Snake River. They've even brought props to help them.

You can see in these scenes the genesis of the Oscar-nominated The Act of Killing, in which Oppenheimer encouraged one particular killer, Anwar Congo, to make a docu-drama about his own past. But while The Act of Killing is full of artifice and the thin veil of fiction, The Look of Silence is unflinching, seeking the clearest possible lens. It was this week named the best film at the Venice Film Festival and has found an Australian distributor, Madman Entertainment, though it's unlikely to be seen in Australia for another year. In Indonesia, meanwhile, some of the anonymous local crew hope that, under new president Joko Widodo, they will be able to seek censorship approval for the film. (The Act of Killing was never submitted to the censors for fear that screenings may be violently disrupted.) Adi says wide distribution would be a start, but not enough in itself. The government needs to start a truth and reconciliation process, he says. "If we ignore it all the time, the same things might happen again. We need to clearly state, 'This was right and this was wrong'." He wants the truth taught in schools.

Reconciliation, though, is a long way off. Adi revealed that, a few weeks after the film finished shooting in 2012, he and his family were visited by some of the old killers. "They threatened us … I got out of [the village]," Adi says He now lives with his family in another city. Even his wife, though, who was initially sceptical about what he was doing, supports his decision to confront these men, some of them who were quite literally bloodthirsty. "We had no choice," Adi says. "Someone has to sacrifice themselves if they want to find the truth."