The strange reason behind why Harry Styles was cast in Dunkirk

Harry Styles was cast in 'Dunkirk' because he has an "old fashioned face", director Christopher Nolan has revealed.

The One Direction heartthrob made his movie debut in the World War II epic and Nolan, 47, decided to take a chance on the pop star because he had a look which suggested he could have lived in the 1940s when the battle against the Nazis took place.

Harry Styles (left) in Dunkirk. Photo / Supplied

Speaking to the Daily Mail newspaper, Nolan said: "He has an old-fashioned face ... the kind of face that makes you believe he could have been alive in that period. Harry's character's very un-glamorous. It's not a showboating role."

Explaining how the 'Kiwi' singer came to his attention during the casting process, he added: "Harry sent in a tape, and we liked the tape. And he joined the workshop, and that was that. It was a really old-fashioned process - and Harry's features, ability and demeanour fitted right in."

The filmmaker - famous for creating 'The Dark Knight' trilogy of Batman movies, starring Christopher Nolan - very much respected Harry, 23, for his work ethic and "humble" personality on set, despite his global fame.

Nolan added: "Harry shied away from being a 'star' in it. He's a humble guy who didn't want attention. He enjoyed the idea of not carrying it; that he was part of an ensemble. Like the soldiers they played, the actors were all in it together and supported each other very well."

Harry Styles in the war film, Dunkirk. Photo / YouTube

Nolan would definitely be up for working with Harry again on a movie, if the young star decides he wants to keep acting.

The 'Inception' helmer said: "Harry's very talented. I've no idea whether he wants to pursue acting, but he's a natural."

'Dunkirk' - focuses on the British-led rescue mission in 1940 in which 330,000 Allied troops were rescued from the clutches of the Germans - has an all-star cast also comprised of Tom Hardy, Mark Rylance, Sir Kenneth Branagh, Cillian Murphy and Fionn Whitehead among others.

The war drama has been nominated for three Golden Globes; Best Original Score, Best Motion Picture - Drama and Best Director for Nolan.