Cops on the trail of a mysteriously disappeared person. The odd couple thrown together, trying to get along. An odd couple of cops, in fact, on the trail ... etc.  The elements of The Keeper of Lost Causes are so familiar as to look like a series of ticked boxes, with the fact that one of those cops is a Muslim providing a bonus tick for topicality.

Following the international success of The Killing and The Bridge on television, there is a ready-made audience out there for Scandinavian crime thrillers. But there has to be a good reason for a maverick production house like Zentropa – home of Lars von Trier and the Dogme movement – to add to the world’s store of cop buddy movies.

“It’s a good question,” agrees Nikolaj Lie Kaas, who plays the taciturn, grizzled old-style detective Carl Morck. Morck, true to type, has ruined his private life and is now married to the job. The Keeper of Lost Causes is based on the first of a series of novels by Jussi Adler-Olsen that is hugely popular in Scandinavia and Germany; director Mikkel Norgaard had made it clear that they must honour the originals, even as they diverged from them. “So we would think about that every day. How are we going to approach this? Why is it necessary for us to tell this crime story once again?”

Their clear focus, he concluded, became the relationships between characters who were considerably more complicated than their one-line definitions would suggest. “For instance, this guy is some even more f---ed up version of Colombo, who is not able to communicate that well. He is not a guy who is interested in being the lead character in a movie. He just wants to be left alone and not be followed around by anybody.”

Kaas is a popular actor in Denmark, known – according to Norgaard – for his charm and winning smile. But Morck never smiles.

It felt risky, remembers Norgaard, to make him so unlikeable. “There were days when I went home saying ‘Am I really killing this?’ Are people going to hate it, like really hate him, or are they going to see the small stuff he’s doing?”

For Kaas, it was a daily acting challenge to make any kind of relationship bloom between Morck and his new sidekick Assad, played by Swedish-Lebanese actor Fares Fares, in such notably stony ground. “When we started shooting, Fares hated it because I was giving nothing to the scenes,” he recalls. “I was just like flattening the tyre all the time – but that was my agenda, because I saw that he was coming with all the energy.”

Fares laughs and agrees. “It wasn’t like we were fighting, nothing like that,” he insists. “It was more frustrating, wondering, ‘How do we do this?’. If someone reacted that way [to me], I would just walk away.”

What he was not prepared to do, he stipulated from the moment Norgaard asked him toconsider the role, was play a character defined by ethnic difference. It’s a rule he made for himself in Sweden, not to play a social problem. “I wanted to be a person,” he says. “It’s up to you to see his differences; I don’t want to play those differences.”

In the books, Kaas says, Morck is an instinctive racist. “I think the film is more interesting in that way,” Fares says, “because they’re equals. In the book, Assad’s a bit of a slave, serving Morck the whole time.”

Norgaard is primarily a television director. He directed much of the Danish political series Borgen. When he was asked to direct a script by Nikolaj Arcel he leapt at the chance to take on a classic genre but, at the same time, worried that the Scandinavian crime film might already have peaked. Would anyone want to see it?

But this is a very Danish story, he thinks. “The sarcasm, Karl’s irony, is such a huge part of the whole Danish approach to humour and to character building.”

He worried even more when they were shooting the film and made a collective commitment to resist any temptation to blur its spiky, Scandi edge. “I was so happy when we edited it and I saw it for the first time and thought 'Phew, we kind of made it'. We had kind of decided: we know what we are going to do. At the end we will see if it works out and if it doesn’t, someone will fire me, but at least we tried.

"I love when you can have an energy like that, because then you can pull something off where you say, ‘It worked out amazing, we tried something here’.”

And that’s the only reason you make yet another cop buddy movie, concludes Kaas. “You embrace the genre in order to change it.”