Picture an African child soldier clad in a white wedding dress and long silk gloves holding his rifle aloft in a victory march. The house his army has raided burns in the background; among them is a new recruit, recently forced into killing his father. At night, these self-proclaimed pubescent “death-dealers” dance around campfires in looted fancy-dress outfits – huge butterfly wings, massive disco-wigs and Evel Knievel-style helmets abound.

Surreal as it sounds, the authenticity of such scenes in Johnny Mad Dog can hardly be questioned. This is the first film shot in Liberia's former war zone, and its young stars were once real child soldiers. Writer-director Jean-Stéphane Sauvaire claims this masterstroke of casting was cathartic rather than traumatic for the novice actors. “Some NGOs use art therapy to help child soldiers express their experiences," he says. "That could be painting, music, theatre – or cinema. For these boys it goes like this – you fight in the war, the war ends, no one takes care of you, you're living on the street. This film was an opportunity for them to express everything. They made it more realistic than I could have imagined.”

Sauvaire says he wanted to bring an upfront, documentary-style to his adaptation of Emmanuel Dongala's 2005 novel. The results are so urgent and relentlessly intense that you sometimes have to remind yourself to breathe. “In a country engaged in war, the temporality is different to normal life,” he explains. “You're always alert. You're never comfortable in your seat watching this film. You never know exactly what's happening.”

For a year, Sauvaire shared a house with 15 of the boys, work-shopping scenes and getting them used to the camera. One night, they watched the Oscar-nominated Blood Diamond; Sauvaire thought it was pretty realistic, but the boys dismissed Hollywood's portrayal of child soldiers. “In the source material for Johnny Mad Dog, one character's name is Rambo,” adds Sauvaire. “When we were preparing our movie, I proposed different names from the book, to see which actor wanted to take each one. I said, 'Who wants to be Rambo?' Everyone laughed. They said, 'Rambo is Hollywood – he never dies, nobody can kill him. We don't want that name. We are real soldiers. We know how it is.'”

Johnny Mad Dog out now.