Actor Dominic Cooper became so "absorbed" in his new film role as Saddam Hussein's eldest son Uday, he found himself behaving more and more like the sadistic heir in his personal life.
The Mamma Mia! star takes on twice the work in The Devil's Double, starring as Latif Yahia, an Iraqi man who was forced to act as the body double of troubled Uday Hussein, also portrayed by Cooper.
But the British actor admits he took his method acting a little too far, and it was up to his brother to help him snap back to reality.
He tells WENN, "(I kept thinking), 'How can I play this hideous man?' There was something I had to truly understand about the mentality of the man to allow me to actually portray him because there's always an aspect of yourself (in the role) so I had to find something humane within him.
"I couldn't do that. I just found him despisable (sic) until I looked into the root of this insane monster and the relationship with his father, who was a dictator, and the awful relationship and the things he was exposed to as a young man; the scenes of torture and the damage that was created was my key in...
"It did change me because I was so absorbed and looking through his eyes constantly. There were moments in my behaviour, I'm sure, where I was becoming slightly more erratic and demanding. If I'd be in a restaurant I'd sense it and just have to monitor it all the time. The rest of the cast would say, 'Dom, you're being a bit Uday...' My brother was on set with me, which was a great leveller. He wouldn't allow me to behave in any other way."
And Cooper reveals the fake teeth he had to wear to play Hussein helped him get straight into character.
He adds: "Once I was on set I was very aware of who I was playing because they (Hussein and Yahia) were so unbelievably different. I understood the difference between their physical gestures and I had these fantastic prosthetic teeth which immediately changed the physical sensation I had. The moment I had those teeth in my mouth I felt the manic, depraved monster and when they came out I felt this warm, solid generous human."
The Devil's Double is based on Yahia's own book of the same name, which details his experience as Hussein's decoy.
Uday Hussein was killed in 2003 during a raid by U.S. forces on a home in the Iraqi city of Mosul.