The 46-year-old director revealed that the former One Direction member had gone through a proper audition to get "Dunkirk", reported LA Times.
"When we put the cast together, we had some established names. But for the guys on the beach, we really wanted young unknowns. He's not that unknown, but he'd never done anything as an actor before.
"So he auditioned. I auditioned literally thousands of young men with different combinations of young men. And he had it," said Nolan.