Christopher Nolan didn't use any green screen while filming 'Dunkirk'.

The 47-year-old filmmaker helmed the epic World War II blockbuster and he admitted he wanted to make the film ''real'' and decided against using CGI and opted for real ships, explosions and planes.

Speaking at the Toronto International Film Festival on Sunday (10.09.17), the filmmaker said: ''When those boys are out there on those beaches and explosions are going off, they're going off.

''There's no green screen. They're in it.''

Although known for CGI heavy-movies including 'Inception', Nolan admitted the CGI graphics wouldn't have been as good for this WW2 epic.

He said: ''You could make a period perfect CG version of a ship, but it wouldn't feel as real. We felt that the match-up, the patina that computer graphics have is a very poor match for this kind of imagery from World War II.''

Previously, Nolan revealed he 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.

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.

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 previously 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.''