Dominic Cooper has admitted he struggled to get into the mind set of playing Saddam Hussein's son Uday as he despised his character.
The Mamma Mia star attended the premiere of new thriller The Devil's Double in London with Uday's former body double Latif Yahia.
The film is based on the true story of Yahia's life as the 'fiday' to the notorious and sadistic Black Prince in Iraq, and Dominic plays both men.
The actor said: "Getting into the mind set was very hard because I despised Uday and I could find no redeeming quality, so it was about understanding what made that man the way he was.
"Because there are truly no redeeming qualities, no moral code - so you really have no idea how someone functions like that.
"I looked at things like his relationship with his father and the fact that Saddam never thought him capable of any military stance, and certainly wasn't going to offer him the role of taking over power.
"So you understand the aggression and the hatred and the bile. That gave me at least a key to understanding the psyche.
"Not that it excused it but at least it gave me an understanding of who that man was."
Yahia's family's lives were threatened unless he agreed to live with Uday in Saddam's palace and appear in public for him if his life was thought to be at risk.
Yahia said it was "very hard" for him to tell his story and he has been tested by many different intelligent services to verify his account.