Everyone loved John Michael McDonagh’s first film, The Guard, with Brendan Gleeson as a sloshed cop. They admired his second, Calvary, in which Gleeson played a priest reconciled to his own murder. His third, a black comedy with Alexander Skarsgård and Michael Peña, was mostly loathed. McDonagh anticipated these reactions, he says in a pub in south London. He had assumed people would like his first two, “and War on Everyone was meant to be divisive”.

So he would be forgiven for having felt perky before the premiere last year of his fourth film. “I love watching the film!” he says. It’s an old-fashioned noir: tense, starry, good-looking. “So I was like: Everyone’s going to love it.” He puts down his pint and laughs.

The Forgiven stars Ralph Fiennes and Jessica Chastain as David and Jo Henninger, an embittered surgeon and a bored children’s author, who have journeyed from Chelsea to Morocco for a friend’s lavish party. It’s late. They’re driving through the desert, lost and bickering and worse for wear. A teenage boy suddenly steps into the road, holding out a fossil he hopes to sell them. David accidentally runs him over.

They put the body in the back and drive on to the party. The police aren’t interested when they are called but the next day the boy’s father turns up and asks David to return with him to his village for the burial. David is reluctant, but agrees; Jo, meanwhile, stays on to amuse herself at the do.

When the film screened at the Toronto film festival last September, critics were confounded. “A lot of the reviews got caught up in how unlikable Jo and David were,” says Fiennes, over the phone. “Therefore: why should we waste our time with these people? That seems quite a simplistic reaction. I think John is making quite a moral film.

“He pushes all the offensive comments, yes. The disparaging, contemptuous attitude wasn’t compromised on, which I liked. But he’s not interested in celebrating; he’s pointing the finger. Some of the responses didn’t seem to be tuned in to the moral journey. Got a bit sidetracked by the louche behaviour.”

Such snagging was not confined to the film, says Lawrence Osborne, who wrote the 2012 novel on which the film is based. Amazon reviews often made the same point. “They say: ‘That was so boring. There’s no one likable,’” he says over video call from Thailand. “You think: What the fuck are you talking about?!”

Matt Smith plays the host of the party: a languorous antiques dealer called Richard who has moved to a castle in the desert. There, he is waited on by an army of servants, along with his vile stylist boyfriend, Dally (Caleb Landry Jones). Richard is a slippery fish, full of perverse choices, but also capable of astute cultural diplomacy.

Smith loved the film’s provocation, he says. It reminded him of seeing Sarah Kane plays in the 90s. “You were arrested by these ideas: ‘Wow! Fuck! It’s in my face.’ That’s what I like. With John there’s such wit and beauty; his knife is very subtle, but he wields it. People say: ‘It’s about all these horrible people and aren’t they awful?’ But these people exist and isn’t it our duty and our responsibility to show them?”

So why did critics disagree? McDonagh has his theories. “Has Marvel infantilised audiences?” he asks, rhetorically. He watches the films too, of course, “when I’m drunk on a plane on a small screen, to give them the level of attention they deserve”.

Just as superheroes tend to remain psychologically consistent, perhaps mortal characters must now follow suit. “Once you’ve introduced a character who says obnoxious things, there can never be any fluctuation. It makes American film critics – maybe audiences – feel uncomfortable. They want a smooth journey. Whereas in real life, we all know that we change our minds the next day.”

That’s what “basically killed” the legacy of his brother Martin’s film, Three Billboards Outside Ebbing Missouri, says McDonagh. Many balked at what they felt was an overly redemptive arc for Sam Rockwell’s dodgy cop. “You cannot make a film about a racist character who behaves vulnerably.”

This stuff matters, he says: “It’s not really fine.” People who reject works of art in part because they find its characters repellent are “kind of in charge of the narrative at a certain point. Which we accept as film-makers, but let’s have some common sense,” says McDonagh.

Osborne, too, sees reaction to The Forgiven as an indication of something more troubling. “Our culture has become much more complex and wealthy, but less sophisticated in its idea of human beings. It’s become more sentimental and crude and therefore less realistic. I think this is very dangerous.”

The problem, he believes, is that “everybody now projects their own ideology into whatever they see”. So if a character is unpalatable, they must simply be dismissed. To engage even with fictional monsters is increasingly iffy.

And it is possible, I think, to detect an element of performative puritanism in how people processed The Forgiven. That would explain why write-ups have tended to use broad brushstrokes to describe the Moroccans as saintly, missing – or ignoring – the fact that the fossil was a decoy: the boy had a gun and was planning a carjacking. When David suggests this in the film, he is dismissed as a bigot. Which he is, of course – but he is also, in this case, correct.

Plus, adds McDonagh, critics kept referring to the boy and his father as Arabs, not – as they are firmly described – Berber. “So they’re accusing you of insensitivity and then they don’t even know that. Hollywood has spent the last 50 years casting Moroccan actors as terrorists or victims of the US military. I assumed naively that if I made a film with fully rounded Moroccan characters who, from early on in the film, control the narrative, people would go: ‘That’s great.’ But no, no, no.”

“Scumbag racist twits are not the worst,” says Osborne. “It’s the white liberals for whom the Moroccans absolutely don’t exist.” Energetic virtue-signalling obscures people’s own failings, he thinks, and the subtleties of those they seek to champion.

“I don’t think the British understand the extent to which they’ve been colonised by the US in this respect. It’s a sort of orgy of Protestant guilt. But you can’t build a culture out of non-stop moral hysteria. And I think its effects will be quite long-lasting.”

Osborne has been an expat for 40-odd years. He specialises in novels about naive westerners disrupting cultures they don’t understand. McDonagh tried to option another of his novels, 2017’s Beautiful Animals, about two moneyed young women holidaying in Greece who give shelter to a refugee. He didn’t win the rights, and now worries for those who did, in light of the reception to The Forgiven.

Both men are robust sceptics who have, at least partially, renounced the Christianity in which they were raised. Small wonder McDonagh, a former altar boy, would be drawn to a story about how the devout view the ungodly, and vice versa.

“You think you can live without religion,” says Osborne. “You can’t. You just substitute something else.” The people he knows in Morocco, in Thailand, all across the world, he says, “don’t take this woke stuff seriously. They just know that you discarded Christianity and this is your replacement.” Problem is: what to do when you want absolution but have no recourse to confession? “From whom will we seek forgiveness? There’s no God to dispense it.”

This is certainly part of the undoing of David: a godless man in all senses. He’s introduced as a withering alcoholic with a blasted marriage and busted professional reputation (there’s talk of a lawsuit from a patient whose tumours he missed). Yet hints of something else are drip-fed: a lefty streak as a schoolboy; a history as an agitator who provoked entitled friends, then may have gradually bought into his own rhetoric.

Monsters are usually made, thinks Fiennes – that means they can theoretically be dismantled. “If you can own your actions,” he says, “there’s a chance of evolving. I think when people implode, they build a carapace as a defence against the fact that they haven’t owned who they are, or the mistakes they’ve made. When you feel lost, the first thing to do is to push people away.

“Life bruises people who might have started out with idealism. People fuck up and create this defence mechanism, which can be a rightwing posturing. Who are we, really? All of us are presented with our own sort of interior odyssey.”

He sounds bashful. That sounds a bit much, he says. “It’s hard to become fully realised. One meets people about whom you feel: I want to be with this person because they have integrity, while this other person has clearly got issues.” He laughs. “Then you have to reflect: maybe I present something that puts people off.”

David begins as nothing but off-putting. He changes, yet only one person seems to notice: Hamid, the chief flunky at Richard’s castle, forever smoothing relations between his coked-up employers and their revolted staff. Hamid glides about – the one person in possession of a backbone – dishing out canapés and increasingly ominous proverbs (“Piece by piece, the camel enters the couscous”).

The actor who plays him sees the film in a slightly different light to the others. Mourad Zaoui beams into his phone from a cafe in Los Angeles and tells me that David was an addict with a death wish. His flaw was not just his lack of religious faith but lack of “faith in one another, in humanity, in the journey. Nobody has faith in him: not his wife, not his friends.”

Any moral salvation is moot, he says. David, he thinks, failed to grasp an olive branch Hamid extends in a late scene involving a drink. “It was not enough,” says Zaoui. “Because Hamid was not white, Hamid was just the help. He focused on the beer more than he focused on the human being. He just drank that beer.” Goodness, I say. I thought the whole point was that David kept eye contact with Hamid? “Maybe with the eyes. But not with the heart.”

Zaoui smiles benignly and says he is “happy to play a brown character that was not a terrorist, very positive, smart and wise”. No, he says, being a Moroccan actor in LA does not require the kind of careful navigation his character faced in the film: “If you don’t believe in the devil, the devil cannot touch you.”

And no, he says, drinking his juice cheerfully, he’s not troubled by any of the reaction to the film. If some people can’t stomach it, don’t force them. “Vanilla doesn’t make you think, make you feel uncomfortable or make you sweat. But if you are used to eating McDonald’s every day, I’m not going to make you eat Indian food.”

• The Forgiven is in UK cinemas from 2 September