In 1965, the Indonesian military overthrew the government. Dissenters to the new regime were smeared as ‘communists’, and over 1 million of them were murdered. In The Act of Killing, Joshua Oppenheimer follows two former executioners as they re-enact their crimes in a film.

A natural reaction to the premise itself would be disbelief, and there are times in the film when credulity will be strained even further. It’s visceral – expect a bruise on your chin from multiple drops to the floor. But it’s all real. It’s basically the strangest and most disturbing making-of documentary you’ll ever see, that will challenge what you think a person is capable of doing and believing.

Early on, lead executioner Anwar Congo matter-of-factly explains his killing method, with his neighbour playing the ‘victim’, that this is the best way to do it because reduces the amount of blood. He becomes our protagonist, flattered at the very idea, and eager to reinforce the pride he has in his job as a gangster, or as he likes to put it, a ‘free man’.

He’s a movie star. Just for the part, he dyes his greying hair and puts in fake teeth. He recruits the hand of his fat bruiser friend Herman, who was also an executioner, to star beside him. Together they conceive a spectacular epic of destruction, with musical numbers and backgrounded by clear Indonesian bays and sexy girls. They form a more compelling leading pair than any buddy cop movie.

It doesn’t stop after Herman’s neighbour, after attempting to persuade him to re-create a scene of the executioners killing his own father, agrees to play the victim in a shockingly convincing performance, complete with blood-curdling shrieks and copious excretion of bodily fluids. Women and children are encouraged to join in, and even they struggle to discern fact from fiction, wailing as if they themselves were the victims, and not actors.

Meanwhile, we are thinking, how far it can go? Herman seems to find the experience spiritually edifying, using the opportunity to dress in drag. Someone persuades him to run for office, and we follow him as he campaigns through the neighbourhood – the residents are outraged there aren’t enough t-shirts for them for be bribed with. We see his opponent’s rally speech; the audience have all been paid to be there.

The killing-culture in Indonesia is pervasive. The necessity and the justice of the massacres is beyond question, amongst every aspect of society. First there is the Pancasila Youth, a ubiquitous orange-clad militia with thousands of members. It’s the thug class, and its job is to propagandise on behalf of the military regime and step on dissenters. Then there is the media; with a smile, young and beautiful news presenters glorify the genocide of the communist scourge; from his office, a universally honoured newspaper baron ordered specific individuals to be killed. It trickles down to even the children of the victims – the attitude seems to be “that is the right way of things”.

Although everyone is outwardly accepting or even supportive of the regime and the killings, the bare fact of its horror cannot be hidden for long. An old friend of Congo and former death-squad leader arrives on a plane. Presumably from his style of dress, from somewhere in the west. Although similarly blasé about his past, he seems more enlightened, and under no illusions that what they did was cruel. He’s the only one who bothers to imagine what the victim’s family felt like.

He agrees to join Congo and Herman in the movie, but their conversations plant a seed of doubt in Congo, that seem to revive previously dormant memories and emotions. Oppenheimer has been with his characters for a long time at this point so he is willing to open up a little. They take on his demons quite literally in an adaptation of his nightmares. Herman plays the part of the lip-licking, rotten-meat-licking entity that tortures Congo, allowing him to tackle whatever his complex is too.

Being a great documentary-maker is somewhat like being a psychologist. Merely recording the facts of the event or your character is fine and necessary, as is drawing attention to the patient’s neuroses, but a great documentary seeks the heart of the issue, attempting to exorcise and test it, and in end, enlightening the characters and the audience (patient and Doctor).

Oppenheimer is up there with Freud and Jung in his ability to go deep inside his characters’ psyches. The film is not so much about killing but a case study of the attitude of killing. It’s an odyssey into evil’s deepest recesses. The process culminates in a devastating final scene which places his film amongst the finest and most profound documentaries ever made.