It's a brave filmmaker who tackles a movie on the Holocaust.

Yorkshire-born director Mark Herman didn't baulk at helming a picture based on such harrowing subject matter, nor did he blink at the prospect of bringing to the screen a story about purity in the face of evil. The project he embraced was The Boy in the Striped Pyjamas.

The difference between Herman's film and something like Schindler's List is not just the modest nature of one balanced against the sheer scale of the other.

It is also the small matter of avoiding sentiment while bringing truth and realism to a book about children that is not necessarily a book for children.

John Boyne's novel was published in 2006 to immediate acclaim. It brought a new dimension to the issue of the Final Solution and provided a simple, understated introduction to what it was and why it happened. The book tells of the friendship between Bruno, the son of a German officer given a new job near somewhere Bruno calls "Out-With", and Shmuel, a Jewish boy who lives in a strange settlement surrounded by a barbed wire fence close to Bruno's family's austere new house.

The story unfolds through a child's eyes and in the first person. It is a narrative based on an innocent's point of view and one that winds its way to a devastating, shock conclusion that is as unforgettable as it is horrifying.

For Herman it represented precisely the kind of gear change he was seeking.

Best-known for his mid-'90s hit Brassed Off, he had experienced a rollercoaster career that slowed to a crawl in 2003 with Hope Springs, a Stateside comedy about a ménage a trois that starred Colin Firth, Heather Graham and Minnie Driver.

It wasn't a film any of his fans expected and, in truth, maybe it didn't quite turn out the way he wanted, either. It emerged as slightly overcooked – a studio picture with studio stars that failed entirely due to studio interference. "After Hope Springs, I was looking to write something that was driven by the heart rather than the need to pay the mortgage," says 54-year-old Herman.

"What struck me most was the fact that the film rights were still available. I was a bit surprised by that but, on the other hand, I could recognise why film studios might not be interested in the book. It's so powerful – and it's tricky.

"At the same time I felt that if the film studio could read it as a screenplay rather than a book they might like it. The only way of doing that was to buy the film rights, which I did in 2005. By the end of the year I sent a work-in-progress screenplay to Miramax."

Herman struck a deal that meant the book's uncompromising ending had to be preserved. Having assured Boyne that his book would be treated with respect, he set to work securing his cast. David Thewlis and Vera Farmiga were hired to play ambitious Nazi and compliant wife. For the central roles of Bruno and Shmuel he went through a rigorous process finally settling on Asa Butterfield, 11, and nine-year-old Jack Scanlon. "I don't know how many kids we saw – it felt like several thousand – but Asa Butterfield was on the first day and I was very struck by him," he reveals.

"We used him as a yardstick and stuck with him. Towards the end we started pairing other youngsters off with Asa to see which worked best. As for the atmosphere, they learnt a lot during the shoot. It was important to me they didn't really know too much about the subject matter because it helped with their performances. When I asked Asa if he knew what the Holocaust was, he said 'Wasn't that when people were turned into soap?' So he had an idea.

"We explained a lot. The last few scenes we obviously left until the end of the shoot and their reactions… they were terrified. Not that they knew much about it. We kept them out of the camp set right until the end as well. That was a really conscious decision, and it helped."

A filmmaker who is proud of never having repeated himself, Herman laughs when he considers the road he has trodden from his debut, the Dudley Moore vehicle Blame it on the Bellboy in 1992, through to his devastating adaptation of Boyne's fable. Bridlington-born and long domiciled in York, he shrugs off all attempts to bracket him as a maker of comedies, "message" movies or quasi-musicals like Little Voice. "I think I've always tried to have a mixed bag, as it were," he smiles. "It tickles me that this is from the same stable as Blame it on the Bellboy.

"The life of a director is such that you devote a long time – two or three years – to a project. I felt after Hope Springs that I really wanted to do something that I really, really cared about. Not that I didn't care about Hope Springs. But subjects come along like this that you know you can confidently give a few years to."

His courage in opting to make The Boy in the Striped Pyjamas was matched by his refusal to compromise his vision. He resisted pressure to hire a star to play the Nazi camp commandant – Daniel Craig and Joaquin Phoenix were suggested – and threatened to walk away if the book's sledgehammer finale was tampered with. He won on all counts.

"We tested the film in America and audiences there are used to happier endings. It was very noticeable – they are suddenly confused because it's not going to happen this time.

"I find that very refreshing. It's like we've entered a competition to make the worst ending in movie history.

"I think we might have won."

The Boy in the Striped Pyjamas (12A) is released today.