“Chris” isn’t the man’s real name. He agreed to collaborate with the film-maker Leo Regan only on condition of anonymity. We hear the words he actually spoke in a series of interviews with Regan that lasted more than a year, but Chris is played by the actor Jason Isaacs, more familiar to filmgoers as Lucius Malfoy in the Harry Potter films or the evil British colonel of dragoons in The Patriot. In fact Isaacs’s performance is mesmerising.

He loses himself in his version of “Chris” so completely that I forgot I was watching an actor. It all helps to make Scars one of the most intense and disturbing films that I have seen for years.

Regan, a slight, quietly spoken Dubliner in his forties, has been living and working in London for decades. Over the phone from a shoot in Canada, Jason Isaacs and I swap notes on his intense manner. “Leo doesn’t do small talk,” Isaacs chuckles. “Early in the shoot he was grilling me. He wanted to make sure I didn’t think this was just another job. I felt like I was being eviscerated.” Regan began as a photojournalist, but turned to documentaries specialising in marginal, often dangerous outsiders — drug addicts (Comfortably Numb), for example, or racist thugs (100% White). We discuss what kind of beast the film actually is. A series of interviews re-created by an actor and the film-maker who is, in effect, playing himself? Neither of us can think of a term for it.

It is possible that Scars will be accused of trying somehow to explain away Chris’s extreme violence, even to celebrate it or, worse still, to derive vicarious pleasure from it. This would be a gross misunderstanding. For one thing the attacks Chris describes, though extreme, are nothing like as sick as the stuff concocted in torture-porn police dramas such as Messiah or Wire in the Blood. There is nothing remotely titillating about it. What is absolutely chilling, though, is hearing a man of violence describing so nakedly how he thought and what he was feeling when, say, he sliced up “some c***’s face” with a Stanley knife. In Chris’s world virtually every other male is a “c***”. Those who hate swearing should not watch.

And that’s the point of the film. “I hope it doesn’t sound pretentious,” says Regan, “but I like to feel my way into my subjects. When I started I wasn’t sure where it would lead. I prefer to go through a development process before, rather than working to a commission, just to see what happens. It was quite difficult for Chris to get his head round that.”

Regan came across him while filming an earlier documentary. “When I met him first he didn’t want to know. He was not interested in talking about his life. That attracted me!” Where it led — and this is what makes the film unique and valuable — is to what Jason Isaacs describes as “a kind of breakdown” for Chris. But this was a slow process.

At first Regan wanted only to film Isaacs being Chris, but Isaacs urged Regan to put himself and his questions back in. “The most interesting thing to me was how this man trusted, or didn’t trust Leo — what he wanted Leo to think of him and how much he wondered whether he was being judged. I didn’t believe that this person, who was so reticent and who was opening himself up in this way, would have done so without quite a lot of provocation from Leo.”

The dialogue between Regan and Chris turns into something with elements of therapy or even the confessional, frequently interrupted, as Chris buckles under the horror of some of the things he has done and thinks, probably for the first time, about the terrible effects on his victims. Then comes a kind of terror, a fear of his own nature, as talking about his violence reawakens the feelings he had while perpetrating it.

Both Isaacs and Regan use words such as “traumatic” and “cathartic” and describe the immense risk for a man who has never opened up emotionally before. Eventually Chris begins to find the origins of his own extreme violence in his equally violent father’s example and seems overtaken by a wave of resentment towards his parents, before homing in on an horrific incident in prison which only stokes his rage.

“Violence offered him power, status with his peers and perceived freedom,” says Regan. “In fact he was imprisoned by it.” Despite genuine fear for his own safety, he liked the man. “I once turned down a job following a supermodel about because I couldn’t stand being with her,” he recalls. “I can only make films where I feel some empathy, respond to a person’s humanity.”

For Isaacs the experience has been intense too. Returning jetlagged to his home in North London after six months in America, he pulled the script from a mountain of mail and read through it that night. “I was electrified,” he says.

“By chance I have witnessed a lot of violence in my life. There are thousands of men like Chris out there. There is something humanising about exposing what might seem like a slightly monstrous character, in seeing his fears, his desires and his pain. It humanises all of us.”

Scars, Mon, More4, 9pm

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