Christopher Nolan said it was "very daunting" making 'Dunkirk' because he is British.
The 47-year-old filmmaker helmed the epic World War II blockbuster, which has been nominated for Best Picture at the BAFTAs and Academy Awards this year,
Speaking to Collider, Nolan said: "It was very daunting, for a British person to take on this story. It's a story we grow up knowing. It's part of the national spirit. We talk about the Dunkirk spirit. It's sacred ground. To a certain extent, you have to put that out of your mind and just try to do the best job you can, but there's a moment where I was privileged to be able to sit with some of the veterans who had actually been there. There are very few of them left and they're in their late 90s now, and that came with a very heavy sense of responsibility. One of the stories I was told, by somebody who has since passed on but that I got to sit with, was about seeing someone just walk into the sea, and seeing men just walking into the sea and swimming out. I said to him 'were they really trying to swim out to the ships or were they killing themselves?' And he said that he didn't know, but he knew that they were dying. I just knew that we'd have to show that and try to get that across."
The impressive cast include Sir Kenneth Branagh and Sir Mark Rylance, others are Nolan regulars such as Cillian Murphy and Tom Hardy, as well as newcomers Fionn Whitehead and One Direction star Harry Styles.
And despite previously admitted he didn't have any idea of Styles' fame, he now said the singer "threw his hat into the ring".
Nolan said: "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."