Dyeing for Your Art: Forest Whitaker gained some fifty pounds of weight for the role. Also in a rather literal sense of this, he mentioned in an interview that his actual skin was also colored to better match the real Idi Amin.
For the scene early on where Idi Amin speaks before the cheering crowd of villagers, the filmmakers didn't have to tell the extras to react that way... because they all believed that Forrest Whitaker really was Idi Amin.
The director decided to use local Ugandan children as extras for a scene where James McAvoy's character is giving vaccinations. Of course, the director did not tell the children or their parents/other adults with them that the syringes were just props. Many of the children thought they were really going to get shots, so their apprehension and nervousness is completely real.