Christopher Nolan says Harry Styles ''earned'' his place in 'Dunkirk'.

The One Direction star made his acting debut in the suspense thriller as soldier Alex last year, and the director says from his very first audition he was impressed with how well the 24-year-old singer read the script, and heaped praise on his ''understated and ''truthful'' performance in the movie.

Nolan - who claims to have not known who the 'Kiwi' singer was - told ''To be honest, he just threw his hat into the ring. We were casting unknowns. We were casting guys of the right age, not 30-year-olds to play 25-year-olds. We wanted 18, 19 and 20-year-olds. We'd managed to fill some of the slots.

''We'd narrowed it down to a group, but we didn't have the character of Alex. He sent in an audition on tape, same as everybody else, and I wasn't really familiar with who he was.

''I mean, I'd heard of him. He just gave a great read and we decided to throw him into the mix for these mass auditions.

''Over the course of a week, changing up groups and trying different combinations, he earned his seat at the table.

''I think what he does in the film is truthful, simple and understated. I was really, really pleased.''

Nolan looked to fellow filmmaker Steven Spielberg for inspiration for the movie as he values him as a ''touchstone'' of the genre, and the 'War of the Worlds' director advised him not to do a movie filmed on water after he did 'Jaws', but he went ahead with it.

Speaking about his influence, Nolan explained: ''Anytime you're working in such a familiar genre, you look to the touchstones of it. You look to the things that really succeeded, in a particular way, for how they're going to inform what you do.

''It clarified, in my mind, that I had to view this not so much as a war film, but as a suspense thriller, and have an unseen enemy and a type of tension with the language of suspense, whereby you can't take your eyes off the screen. What Steven did so brilliantly in 'Saving Private Ryan' was use the language of horror, which is one whereby you're looking away from the screen.

''It's a different thing, but it's very useful to be able to talk to filmmaker. I mostly spoke to Steven about 'Jaws' and about shooting on the water because I'd never done that, and his advice was, 'Don't.'''