“I think it’s fair to say,” says Géza Röhrig softly, “that we haven’t learned anything from Auschwitz. The cruelty exhibited there exists today against the Kurds and elsewhere. You have a feeling of insecurity about tomorrow. There’s a level of chaos because global powers do not agree on the most minimal consensus.”

Röhrig is the star of Son of Saul, a tense, almost unbearable thriller set in Auschwitz in 1944 among the Sonderkommando – prisoners given a stay of execution to work in the gas chambers. It’s so frank and unflinching, it makes even the finest of previous Holocaust films look crass. “With movies like Schindler’s List, you have an evil guy and you have a good guy,” says Röhrig. “There’s no such thing. We are all evil and good inside.”

One of the lessons of 1944 lies with the bystanders – we can't just let things happen

Contemporary resonance, though unintentional, is unavoidable. This is a film about whether one participates in the suffering of others. “I gave up thinking that society is anything other than an abstraction long ago. You have different societies in every country. But whichever group you belong to, you’re never exempt from taking a side when it comes to crimes against humanity. That’s true in Syria and America and Israel and everywhere. Every day, we all have to make a case-by-case evaluation: is this an important enough demonstration to go on? Is this where I should send my money? Is this a petition I should sign? I do not believe in living in an apolitical ivory tower. One of the lessons of 1944 lies with the bystanders – we can’t just let things happen.”

Just in case it’s not yet evident, Röhrig is not your average starlet. Son of Saul is his first movie, but at 48 he’s gentle, intense, heavily bearded and unswayed by the glitz. More than that: he’s allergic to it. Such escapism as the entertainment industry offers he holds partly responsible for exactly this apathy. “Consumer society targets your senses and threatens thinking entirely. ‘Why aren’t you laughing? There is so much fun! Why don’t you try this cake or that Coke?’” So Cannes can be hard to stomach? “No question. There is a certain level of shallowness – emptiness even – that is really painful.”

Röhrig was born in Hungary; his father died when he was four and after some years in an orphanage he was adopted, aged 12, by Jewish friends of the family. In high school he spent half a year in Paris, “growing up, so to speak, on the breasts of the enlightenment”, before being expelled at 16 for anti-communist activity and founding an underground punk band, Huckrebelly, which played under different names to dodge the cops. He studied Polish literature then film direction, lived in Jerusalem before moving to Manhattan in 2000 to study at the Jewish Theology Seminary. He now teaches, writes poetry and is working on his first novel, Dead Bread. Five months ago, his wife gave birth to twins. He mostly looks at his phone, he says with a smile, to check they’ve eaten.

Röhrig’s face fills almost every frame of Son of Saul. The director, László Nemes, says he was drawn to the actor because “his facial features and his body are always changing. It is impossible to tell his age, for he is at once old and young, but also handsome and ugly, ordinary and remarkable, deep and impassive, quick-witted and slow. He moves, is given to fidgeting, but knows how to keep silent and still.”

Today, in a windowless, heavily mirrored, incongruously orgiastic hotel bar, Röhrig keeps still, save for the odd impassioned tap of the table. He’s unlike any other actor I’ve met but, at a push, one could liken him to an especially bookish Mark Ruffalo, bearish machismo leavened by slight, nearly reedy voice. In the film, his face is almost calcified, drained of emotion as he goes about his work – herding the new arrivals into the gas chambers, scrubbing down the walls and floors afterwards, stacking the bodies, putting them into the incinerators, shovelling their ashes into a river.

Géza Röhrig, left, in Son of Saul, directed by László Nemes.

At the film’s press conference, Röhrig reacted with anger to one reporter’s suggestion the Sonderkommando were “half-victims, half-hangmen”. Anyone who read their testimonies could not, he says, have “the slightest doubt that the Sonderkommando were not just equally victimised but more victimised. They lived in the epicentre of hell. I think they deserve utmost respect. Some of them tried to make their way into the gas chambers instead.” That “sophisticated journalists” betray such prejudice came as only a mild surprise – the Sonderkommando have long been the victims of such ignorance. “Partly because everything about them was hearsay. The rules of the Nazis meant they couldn’t meet with the rest of the inmates, so they were somewhat mysterious. And the victims had so much anger and frustration, they projected it on to them.”

Such displaced blame may also stem from the logic that if one accepts the Sonderkommando’s guiltlessness on grounds of compulsion, one must also exonerate some Nazis. But Röhrig rejects this, drawing a distinction between the prisoners’ work and, say, “pouring crystals of Zyklon B into a fully crowded gas chamber. I would reserve the right to morally question such action. It is clearly problematic to murder even one person, let alone hundreds, simply because you’re commanded to do so.”

But he does admit wrestling with one aspect of the Sonderkommando’s duties: “To fool the Jews that they’re taking a shower. That is the only question I think has some legitimacy.” Yet, he thinks, to have done otherwise, prompting panic and a bloodbath, would have been as cruel and fruitless as a doctor explaining to a terminally ill patient that a period of euphoria often precedes death. “So I think this lie is justified, as bad as it is. It’s graceful. I don’t think they were obligated to tell the newcomers that in the next half-hour, they’d be gassed to death with their children and parents. For the rest of their functions – moving the dead, burning them, taking their ashes, cutting their hair, searching their dead bodies for jewellery and golden teeth – I see absolutely nothing I think is unethical. It was the only way for them to do their best to survive.”

Watch a promotional clip from Son of Saul (in Hungarian, with French subtitles) Guardian

Röhrig’s relationship with God began before he was aware of religion. He would speak to someone who “for a while, I thought was my father. And then I realised it wasn’t him. I just needed someone to talk to. I think there is a side of us that we can’t share even with the ones we love, and that side has to be aired. It has to be communicated. So I talk to somebody. And I call that somebody God.”

It wasn't God who rounded up the Jews, Gypsies, Soviet PoWs, gays and midgets and slaughtered them. The human family did

His faith was then forged on a trip to Auschwitz. Making a movie set there inevitably tested it. “But it wasn’t God who rounded up the Jews and the Gypsies and the Soviet PoWs and the gays and the perfectly German mental patients and the perfectly German midgets and slaughtered them. We did it. The human family did it. I do not for one nanosecond like to pretend that God is off the hook. He could and should have stopped it at a much earlier stage. But I would not be able to get up from my bed in the morning, let alone pray, if I didn’t fully believe that God somehow was there holding the hands of each and every Jew in the gas chamber – each and every Tutsi, Armenian, Kurd, Israeli, Palestinian who suffers unjustly.”

It isn’t the right of survivors to say God was not present, he thinks, given that those who died are not available for testimony. “So I think there is room for me to believe, as irrational as it sounds, that since God is all-capable, in some mysterious way, he suffered along and was there. If I wasn’t able to believe this, I don’t know why I’d take my next breath.”

Atheism he can respect: his elder son has taken a stand against faith, he says. “And trust me, I did my best to raise him to be some sort of believer.” It’s agnostics with whom he struggles. “We are not living in the same reality. I can’t imagine a more pressing question. Life is so threateningly fragile. I just can’t take their attitude seriously. They bore me.”

Has he ever had proof for his faith? Never. And he wouldn’t want it. That, he says, would force him into a corner and rescind his free will. “I always like to have the option to say no. It’s like love. One day, you go out with a girl and you feel like she’s the one for you.” To prove such love by cutting off a finger is impossible. Yet millions would go much further in the name of a deity. “Faith can never be blind,” he says. “Only idolatry can.” So those millions “are not religious in my vocabulary. They are cult followers.”

Son of Saul is an extraordinary caution against idol worship. And its leading man’s rare consciousness of such fundamentalism informs the whole film. “I do idol worship sometimes,” he says. “But as soon as I realise, I smash my idols into a thousand pieces.”