To embody the father of the atomic bomb, Christopher Nolan cast an actor who has appeared in five of his films, Cillian Murphy.

The appeal and challenge of playing Oppenheimer, Murphy says, was doing justice to the physicist’s immense intelligence and moral struggles. “We were always chasing after the complexity of Oppenheimer, as he was no simple man,” Cillian Murphy says. “None of the people in this movie are. Having that huge intellect can be a burden; people like that operate on a completely different plane than us mere mortals, and that brings its own complications and challenges to their personal life and moral life. That was one of the trickiest things: plotting Oppenheimer’s moral journey through this story, because he’s kind of dancing between the raindrops through a lot of it, in terms of where he stands morally with his job at the Manhattan Project, and then, years after it, where he stands in terms of nuclear policy after World War II, and how his changing, evolving stances put him in conflict with other people.”