Christopher Nolan ''wasn't really familiar'' with Harry Styles before he cast him in 'Dunkirk'.

The acclaimed director has admitted he was only really aware of Harry's fame because of his children - but feels he made a wise decision to cast him in his much-anticipated new drama movie, which centres on the Dunkirk evacuation in World War Two.

He explained: ''As a director, I have to trust my instincts, my ability to figure out who's the right guy for the part. I'm not too worried about baggage.

''I was new to Harry. I mean, I've heard his name from my kids, but I wasn't really familiar with him ... What I was seeing [when he auditioned] was a very charismatic guy who clearly had a truthfulness and a subtlety in his ability to perform as a film actor.''

The decision to cast Harry in the movie was greeted with some scepticism, with many onlookers suggesting it was a cynical ploy to raise interest in the film.

But Christopher has compared the situation to when the late Heath Ledger was cast as The Joker in his widely acclaimed film 'The Dark Knight Rises'.

He told ET Online: ''When I cast Heath Ledger as The Joker, it raised a lot of eyebrows and caused a lot of comment.

''I have to trust my instincts, and Harry was perfect for this part.''

Despite this, Christopher hopes Harry's fame doesn't serve to overshadow his on-screen performance.

The director shared: ''What I'm hoping for, when people see the film, is I'm hopeful that they won't miss what he's done, because it's very subtle, very truthful and real.

''I wasn't giving him a sort of flashy thing to do. But it's really important what he does in terms of what it says, I think, about human nature and what people do in different situations, and I think he pulled it off with incredible grace and reality. And that as a director, that's what you're looking for.''