Playing a concentration-camp commandant in The Boy in the Striped Pajamas—opening here Friday (November 14)—was no picnic for David Thewlis. But donning the swastika and making not-nice on-screen wasn’t as hard for the versatile British actor as one might imagine.

“Oddly enough,” Thewlis explains, calling from Toronto, “we had quite a bit of fun on-set. The atmosphere was often quite humorous and childlike.”

That’s because most scenes of the fablelike movie involved school-age children and it was necessary to keep things light for them.

Adapted from John Boyne’s popular novel by director Mark Herman, the maker of eccentrically small British films like Brassed Off and Little Voice, the tale stars newcomer Asa Butterfield as Bruno, the young, openhearted son of Thewlis’s ruthless officer, with Amber Beattie as his morally pliable teenaged daughter, and Vera Farmiga as his increasingly stricken wife.

“There was quite a nice rapport between everyone on set,” the actor continues, “and we all managed to laugh a lot. We had several dinnertime scenes where the kids would be flickin’ their food at each other.”

At night, however, the Lancashire-born film veteran and occasional writer continued his research into the human cost of organized hatred-hence the title, which outwardly refers to the small Jewish lad (Jack Scanlon) Bruno finds on the other side of the barbed wire. Among other grim things, Thewlis read the memoir of Auschwitz commandant Rudolph Hí¶ss, who raised five children virtually on the grounds of the German death camp.

“I often wondered what happened to those children after Hí¶ss was executed; I read somewhere that they came to Canada, actually.” (Their fate remains unsubstantiated.) “Anyway, shooting was fine, but off-set it was, ”˜All right, let’s fill my head with all this horror.’ I punished myself with some really awful dreams, I can tell you.”

Like Thewlis, and his stoical soldier, the audience is expected to already know enough about what’s happening off-camera to grasp Bruno’s confusing plight.

“John Boyne chose to write it through the eyes of a German commandant and an innocent boy. It’s an allegory about prejudice and confronting what it does to people. This is what attracted me to the project. It didn’t seem like anything else I knew of in the English language that could be applicable to children. It’s not a children’s film, per se, but I think it’s a good place to begin their education. With just a little effort and changes of costumes and names to a different time and place, it could be set in Darfur and have the same messages coming through.”

The tall, craggy-faced 45-year-old, who burst onto the international stage 15 years ago with his harrowing lead role in Mike Leigh’s scabrous Naked, has since learned something about movies aimed at wee ones. He played Prof. Lupin in the last two Harry Potter movies-which, for some strange reason, I still haven’t caught.

“Well, you haven’t missed a thing, mate,” Thewlis concludes with a laugh. “But do try to get people to see this. It’s the feel-bad movie of the year!”

This article was updated on November 14 at 5:08 p.m. A change was made to correct a factual error.