The Brit plays both Saddam Hussein's evil son Uday and his body double in the drama, and reveals he watched Jeremy Irons and Sam Rockwell portray multiple characters in a single film before his shoot.
And Cooper admits he "relished" playing dual leads - because it meant he was a permanent fixture on set.
He tells Britain's Esquire magazine, "I knew it would be hard (to play two characters), but I loved feeling part of it all. You can feel a bit removed on a film set, coming in to do your scene for one day, three days. You're a warm prop, really, which is why I relished doing this so much.
"I watched Dead Ringers, and Moon, which I thought was an incredible film, and I also looked at the brothers in The Social Network. But all those guys are meant to be twins or clones, whereas this is two very different people."