David Thewlis found it impossible to prepare for his latest role as a Nazi concentration camp leader - because he couldn't understand the motivations of someone so evil.
The British actor admits his part in The Boy in the Striped Pyjamas was hard work - but reveals he read history books to familiarise himself with the period in which the film is set.
He says, "Trying to understand someone who was impossible to comprehend was a real challenge. It was difficult to get my head around his motivation.
"I usually base my performances on people I've met, which was impossible playing a monster such as this. I did a lot of research to familiarise myself with the period."