Here was a story begging to be investigated on film. Belgian director Joachim Lafosse’s A Silence is based on the real-life case of Victor Hissel, a lawyer who represented the distraught parents of two of the children raped and murdered by serial killer Marc Dutroux. In a bizarre twist, Hissel himself was then charged with possession and use of child pornography.

It was a huge news story in the mid-2000s, but Lafosse saw that there was a truth to it that could be reached only through fiction. “With a journalistic account, you don’t have real characters,” he says when we meet at the San Sebastian Film Festival, where A Silence became the biggest talking point. “My job is to write them.”

Lafosse wrote several versions of A Silence, from the points of view of one character after another: the fictionalised barrister, his teenage son and finally, his wife, whose determined, infuriating denial of her husband’s crimes is the puzzle at the heart of the film. Daniel Auteuil plays silk Francois Schaar; newcomer Matthieu Galoux plays his troubled son, Raphael, who is about to be expelled from his umpteenth school for truancy; Emmanuelle Devos plays Astrid, Francois’ wife of 30 years. In an extraordinary performance, she manages to convey with tiny gestures, glances and even the rhythm of her breathing a desperate sense of losing control.

A single telephone call – taken in the car as she drives, voices never raised – reveals that, early in her marriage, her celebrated husband abused her younger brother. Her brother now wants to tell the police; he is sick of living with the past. But why do that now, she asks. It was so long ago; Francois apologised! It was a mistake made in the moment. And now he is under so much pressure!

But she has inklings, as does the audience, that it is not over; we see Francois pad down to his study in the middle of the night, hear the shrieks emanating from his laptop.

“This is actually the result of a mental construction over 30 years,” says Devos, also in San Sebastian. “I saw a lot of movies about different stories of this kind of incest, documentaries, and what is incredible is that the mothers ... always react in these astonishing ways. They are always, always in denial. I think you just don’t want to see the horror inside your home.”

Lafosse hopes audiences will empathise with Astrid, who is a product of her class and its anxieties. “I abhor the bourgeoisie and their silence, the comfort they choose by being silent instead of speaking the truth,” he says. But he does not judge her. “It is very easy to condemn a woman who doesn’t speak about horrible things for 30 years, but she is also a victim. I prefer to judge the criminal.” He compares the crime to a poison that seeps into their lives.

That poison was potent enough to deter half-a-dozen actors he approached to play Francois. Each told him it was a great script, but they would never touch it. Then he sent it to Auteuil, who accepted it within 45 minutes of receiving his email.

“I know that in his life, certain situations and certain people he knows very well are victims,” says Lafosse. Auteuil’s former wife, Emmanuelle Beart, has revealed that she was abused in her early teens by members of her family.

“The mothers ... are always in denial. I think you just don’t want to see the horror inside your home.”

Emmanuelle Davos

“Daniel understands it,” says Lafosse. “In fact, he spoke more to me in defence of Astrid than about his character. Because he knows that his character, the lawyer, won’t change. Guys like [that] refuse to change. Their sickness prevents them from asking for help, and they choose the worst way possible.”

Schaar has the arrogance of wealth, education, position and a conviction that he is always in the right, recognised as a public hero standing up for little people – the murdered children – against the indifference of the state. Auteuil brings with him the real-life authority of his fame, as well as his powers of persuasion as a great actor.

“I was confident Daniel would help us,” says Devos. “I thought it would make it believable she loved this man more than anything because Daniel is a very seductive person.”

Writing A Silence consumed Lafosse for eight years. “Sticking with a project for so many years always requires a lot of dedication,” he says. “Writing the screenplay can be all about trying to find the reason why you want to make that film.”

For much of that time, he couldn’t acknowledge that while Hissel’s case was fascinating, it was his own history that made it so compelling.

As he will now say openly, Lafosse was himself a victim of repeated sexual abuse. Like Raphael, he was a teenage school refuser. His initial abuser was a teacher who offered to help him study for his Baccalaureate outside the school system. He was 16. Other men became involved.

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In 2008, when he was 33, he made a film called Private Lessons that was drawn directly from his own experience, but he didn’t admit that. Nor did anyone ask. There were people who saw it who would have recognised the situation, maybe recognised themselves.

“Once the film was released, they crossed the street when they saw me. Nobody approached me to talk about what I had shown.”

His own mother, in a way that finds its echo in Astrid’s quiet desperation, never talked to him about it. A friend warned her against seeing Private Lessons; she avoided it for another seven years. Finally, she saw it and called him.

“And she was wonderful. She said: ‘I am sorry; I am with you; you are not responsible and I understand very well what has happened.’ She asked me if I would like to go to the police, but I said no. That I had made a movie, that I had been doing psychoanalysis three times a week for 25 years to understand what had happened to me and that I had thought a lot about it, for me, it was too much.” He supposes now that he just didn’t have enough courage.

Gradually, Lafosse realised that it was the reaction to that film – not least his own reaction – that had led him so inexorably to Hissel’s story.

“I understand now why finally I did this movie,” he says. “It is about the shame – and the consequences of the shame.” Every crime, he says, creates its own closed world that locks its initiates into a collective silence. “If you speak of financial crime in a bank, there are people who know what was done, a group living with that, outside society. A sort of world within the world.

“And what I discovered during the shooting with Emmanuelle was that this was about the silence around shame. Why you don’t speak, why you don’t ask for help. The way that when it first happens, you refuse to believe it. And if, after a long time, you accept what is in front of you, it’s too late because you didn’t speak. Time has passed and now you are the guilty one because you didn’t speak. It’s a vicious circle.” With this film, he has set out to break it.

A Silence is in cinemas from June 27, with advance screenings from June 21-23.

If you or anyone you know needs support, you can contact the National Sexual Assault, Domestic and Family Violence Counselling Service on 1800RESPECT (1800 737 732).