Christopher Nolan looked at firsthand accounts of World War II survivors for 'Dunkirk'.

The 46-year-old filmmaker researched by watching real life footage and reading records from the British-led rescue mission in 1940 in which 330,000 Allied troops were rescued from the clutches of the Nazis.

Speaking at the world premiere of the movie London's Odeon on Leicester Square on Thursday (13.07.17) evening, he told reporters: ''I was looking at a lot of firsthand accounts of people who were there.

''I want people to see it in the big screen and feel like what it was like on the beach or be in small boat.

''I wanted to make the film feel real. I watched archive footage of what it was like. It was about realism for me and staging it in front of a camera rather than green screen.''

Nolan's producer wife Emma Thomas, who also attended the premiere, said the story of Dunkirk is something everyone who grows up in Britain knows about.

Many of the actors in the film are well-known including Sir Kenneth Branagh and 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.

Thomas said: ''Casting was a very long process but one of the more fun casting we had. We managed to have roles for established actors, Kenneth Branagh and Mark Rylance.

''We got to work with people we have worked with before as well as discovering new talents. And they were all fantastic.''

The idea of realism and authenticity was something important for Nolan who even made Styles change the way he tied his shoelaces as soldiers would have done it a specific way.

Styles said: ''When I heard about Chris doing it I was kind of already excited to watch it to be honest and I just wanted to be involved...

''On my first day he told me my laces were tied wrong.''