Anwar Congo and his grandkids from the film “The Act of Killing.” (Drafthouse Films)

Here’s how you interview Joshua Oppenheimer: Turn on recording device. Ask question. Allow approximately 55 minutes to one hour for the answer, which will cover everything from his new documentary, “The Act of Killing,” to the nature of evil, the myth of “fly-on-the-wall” filmmaking, globalization, pre-CGI special effects and the psychology of trauma.

Turn off recording device.

Oppenheimer, an intense, compactly built man of 38, has come to Washington to talk about “The Act of Killing,” but the visit is also a homecoming: He grew up here, first on Capitol Hill, then in Takoma Park and finally in Chevy Chase, the son of a University of Maryland political science professor and labor lawyer. “This is home,” he says at the downtown office where he’s been doing interviews.

A few moments later, Oppenheimer has launched into a long, neatly organized chronology of how he came to make “The Act of Killing,” a film that recounts the mass murders of communists, ethnic Chinese and intellectuals in Indonesia in 1965 and 1966 and documents the massacres’ pernicious aftermath. The film, which was given its premiere last year at the Telluride Film Festival, has been a sensation on the festival circuit for its artful fusing of cinematic artifice and truth at its rawest and most unnerving. Oppenheimer not only found a death squad leader and his followers who would speak candidly about the crimes they committed 50 years ago, but they also reenact those episodes, in lurid, amateurish improvisations derived from the Hollywood genre films they imitated during their most heinous actions.

The result is a chilling account of slaughters that still reverberate throughout Indonesia in the form of corruption, cynicism and fear. The film also operates as a surreal meta-meditation on the grammar of violence, the legacy of co­lo­ni­al­ism, self-deception and the possibility of remorse. For Oppenheimer, it all started with the simple question of what happened in Indonesia in the 1960s, which he started asking on his previous project, “The Globalization Tapes.”

Filmmaker Joshua Oppenheimer's new documentary deals with the death squads that patrolled Indonesia in the 1970s. (Marvin Joseph/The Washington Post)

Oppenheimer was living in London, having studied film at Harvard and as a Marshall Scholar, and he was working with former classmate Christine Cynn on developing experimental documentaries. Oppenheimer and Cynn were commissioned by the International Union of Food and Agricultural Workers to make a documentary in a country where workers were trying to form a union, and the team found itself in Indonesia, filming workers on an oil palm plantation. “I didn’t know Indonesian yet, I didn’t know about the killings,” Oppenheimer recalls. “And it turned out the biggest obstacle that these women had in organizing a union was fear.”

Oppenheimer discovered that many of the workers’ parents and grandparents had been in unions in the early 1960s and then were accused of being communist sympathizers when the military dictator Suharto came to power. “They were placed in concentration camps by the army in ’65, and then dispatched out to civilian death squads that would take them to riverbanks and kill them,” Oppenheimer said. “And [the people we interviewed] were afraid this could happen again.”

After completing “The Globalization Tapes,” Oppenheimer and Cynn returned to Indonesia almost immediately, “knowing this was a very important history that we didn’t really understand.” As they conducted interviews, survivors urged them to speak to the killers themselves, who were living not just openly but also proudly with their brutal history. What Oppenheimer discovered was that the killers — who call themselves “gangsters” — still operate with impunity in Indonesian culture, bragging of their past deeds and continuing to intimidate their victims’ descendants. (Cynn eventually left the production, and Oppenheimer worked with several anonymous Indonesian crew members, including his co-director.)

“My questions started to shift from ‘What happened?’ to ‘What is the function of this boasting?’ ” Oppenheimer recalls. “Why are these men boasting? What is the effect on their society? Are they trying to convince themselves of something, convince me of something? How do they want me to see them, and how do they see themselves?”

If “The Act of Killing” has a star, it’s Anwar Congo, a lean, gray-haired former death squad leader in North Sumatra, whom Oppenheimer met after interviewing dozens of perpetrators. For their first interview together, Congo took the filmmaker to a rooftop where he used to dispatch his victims. He uses a companion to demonstrate how he garroted people with a piece of wood and some wire. Even on that first day, Oppenheimer noticed, “Anwar had thought to bring a friend, and he’d thought to bring wire.”

Congo turns out to be a bizarre protagonist, compelling and even engaging as a guide through Indonesia’s darkest chapters, repellent in his self-justification and lack of empathy, pathetic in his bravado. At one point on the roof, after he reenacts the murder, he begins to dance, explaining that he often went out partying after killing people.

“When Anwar dances on the roof, I was shocked, I was outraged,” Oppenheimer says, recalling the moment. “I saw an allegory for impunity that would expose the nature of this regime just as I had been looking for on behalf of the survivors and the human rights community. But at the same time, I think there’s a stone in his shoe when he’s dancing the cha-cha-cha. He says he’s a good dancer because he was drinking, taking drugs, going out dancing to forget what he’s done. So I think he was shadowed by his past.”

It was Congo’s idea to stage the reenactments, in which he and his fellow squad members relive the torture they delivered as noir-esque gangsters and Western cowboys. In a musical sequence that appears in snippets, dancing girls snake out of an empty seafood restaurant designed to look like an enormous fish. In between those sequences, death squad leaders walk through a marketplace in the north Sumatra city of Medan, shaking down Chinese shopkeepers, later laughing about their past deeds with government officials.

The net effect is akin to a fever dream that swiftly slides into the stuff of nightmares. But Oppenheimer insists that “The Act of Killing” doesn’t just capture an isolated area, geographically or morally. “This is an extreme example of the stories we tell ourselves to justify our actions,” he says. “I’m wearing a T-shirt that my partner picked up in New York at H&M for six dollars. And I read the tag and it says it’s made in Bangladesh. And I’m thinking, ‘Is the person who made this T-shirt for me dead? In a pile of rubble?’ In a way, everything we buy is produced in places like the reality you see in ‘The Act of Killing.’ . . . In that sense, we depend on Anwar and his friends to help keep everything we buy cheap, and we know it. So this is the dark underbelly of our reality. It’s not a distant reality.”

Oppenheimer, who’s now based in Copenhagen, says that “The Act of Killing” has been well received in Indonesia, where last fall, the weekly news magazine Tempo devoted an entire issue to the film and conducted its own investigation of the killings. As of April, the filmmaker says, the film had been screened 500 times in 95 cities “and every week now there are more screenings than there were the previous week. And in due course, we’ll make the film available for free download and free screening for anyone logging on in Indonesia.”

His hope is that “The Act of Killing” will be the first step on a long road toward a public reckoning, with truth commissions, trials, reconciliation rituals and a presidential apology. He quotes Werner Herzog, who with Errol Morris executive-produced “The Act of Killing,” after Herzog finished reading the Tempo magazine edition. “Werner said to me, ‘Josh, art doesn’t make a difference.’ And I felt so sad when he said that,” Oppenheimer recalls. “And then he paused and smiled and said, ‘Until it does.’ ”