Monos is set in a South American paradise but comes off like Lord of the Flies in hell. Following an eight-strong group of teen soldiers and their adult prisoner of war, it is sensual, and gorgeous and intensely, continuously disturbing. The kids – Dog, Lady, Bigfoot, Boom-Boom, Rambo, Swede, Wolf, Smurf – are left to their own devices except for the occasional visit from “the Messenger”, who works for whatever organisation has recruited them. So they scrap and kiss and fire their AK-47s into the sky.

They met a man with dwarfism who became a child soldier at 11. 'He was an amazing destroyer of things'

Alejandro Landes, the film’s 39-year-old director, says it doesn’t matter who they’re fighting for. “Colombia’s been war-torn for so many decades. I’ve seen that violence happen from the left and I’ve seen it happen from the right. My grandfather’s favourite place in the world, his farm that he worked all his life to buy, became the headquarters for the most gruesome rightwing paramilitary group, the AUC, and I’ve had part of my family kidnapped by guerrillas.”

Who, exactly? “My grandfather’s sister, my cousin … things like that.” Were they freed? “Some yes, some no. So that is something that I had seen and experienced.” These experiences inspired and informed Monos.

We’re given no specifics of time and place – it’s somewhere in Latin America. Landes, who lives in New York, has Colombian roots but grew up in Ecuador, his parents having left Medellín in the 1980s to escape the violence. To find his actors, he and his producers sifted through 800 tapes, selecting 25 possibles totake part in a mock training camp for four weeks in the Andes. Most had never acted before, though there was one major exception: Moisés Arias (Bigfoot) was a regular in Miley Cyrus’s Disney series Hannah Montana. One they saw on a tape playing basketball, a kid with a close crop, referred to on the court as Matt, but in fact named Sofia (Buenaventura). She plays Rambo in the film and, says Landes, “was very inspirational for the fluidity of the character”. He calls the character, who is written as male, “post-gender”.

Facebook Twitter Pinterest ‘One morning he woke up and couldn’t move.’ Alejandro Landes. Photograph: EFE News Agency/Alamy Stock Photo

Researching the film, Landes visited Colombian reinsertion camps, meeting adult rebel fighters who had laid down their guns and wanted to enter civil society. In one he found Wilson Salazar, a man with dwarfism who became a child soldier at 11 and deserted when he was 24. There was a sizeable price on his head, dead or alive. “He was very good at what he did,” says Landes. “He had risen to be part of a column called Teófilo Forero, one of the most feared combat units. He was an amazing destroyer of things.”

Landes hired Salazar as a military consultant, then asked him to play the Messenger, too, as he was so convincing. At the camp, Salazar woke his young charges at 4am each day. They’d undertake acting exercises in the mornings, and in the afternoons Salazar would give them clandestine military training. “It was intense,” says Landes. “It was a boot-camp scenario – the kids were living, sleeping, eating together, everything together, all at the same time.” Landes then selected his eight, tweaking the screenplay to fit.

The shoot was yet more full-on. The opening mountaintop segment was shot at almost 14,000ft (4,300m): punishingly cold and wet. “You move slower up there,” says Landes. “There’s low oxygen, you move in a bit of a daze.” On the first day, someone from the camera department had an epileptic seizure. A baptism by fire, thought Landes; things will get easier. They did not. For four weeks, the 55-strong cast and crew filmed in the jungle around the Samaná river, five hours south-east of Medellín. Mules carried their food; Colombia’s national kayak team helped with filming on the rapids; a family of local gold miners built their military tents. There was no electricity, no running water, no refrigerated food and torrential downpours all night every night. There were food rations. The shoot took its toll.

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“People were dropping like flies, including me,” says Landes. “Everyone had their day. Everyone cried on this movie. You’re working six-day weeks, very high-intensity scenarios.” Landes himself was working seven-day weeks. One morning he woke up and couldn’t move. The gold miners put him on a stretcher and carried him up a canyon until they reached a local medical centre. He cried on the journey, distraught that the production might collapse. He was back behind the camera 20 hours later. “I think it was just the stress,” he says. “And the ration diet.”

It was worth it. The film has earned universal raves – and a special prize in San Sebastián film festival, Spain, for accurately reflecting “the values and reality” of LGBTQ people. The story is a nightmare in which nothing – gender, sexuality, political allegiance – is muddled or irrelevant. “It’s not binary. That’s why I think it reaches much further than Colombia. That is what’s most timely and relevant. I wanted to make sure that the film spoke not to the moment but to us as a species.”

• Monos is released in the UK on 25 October