'I threw myself into reading a lot of first hand accounts by people who've been there, a lot had been compiled by the imperial war museum. Joshua Levine had compiled a book called Forgotten Voices of Dunkirk, he came on as a historical advisor. I spent a lot of time talking with him, reading materials that he was able to find me. Then we were able to have the great honor and privilege of actually going and speaking to people who had been there. Obviously at this point, veterans of Dunkirk are very old and there are not that many left, but some of them very graciously gave us their time, and we were able to actually talk to them about what it was like to be there. One of the stories that stuck in my head and worked it's way into the film was a veteran telling me about watching people just walk into the sea, just as if they're going to swim. I asked him were they literally trying to swim back to England or swim out to the boat, were they killing themselves? He didn't know. He knew they were gonna die. It's a chilling thing to hear.
My pitch to Warner Brothers was, we're going to put the audience into the cockpit of a Spitfire and have them dogfight against the German Messerschmitts. We're going to put them on the beach, feeling the sand getting everywhere, confronting the waves. We're going to put them on small civilian boat bouncing around the waves on this huge journey heading into this terrifying water. It’s virtual reality without the goggles. I knew that I didn't want to make a film that could be dismissed as old-fashioned, something that wasn't relevant to today's audiences. What that ruled out for me immediately was getting bogged down in the politics of the situation. Seeing the generals in room. Seeing Churchill. We don't have Generals in rooms pushing things around on maps. We don't name the enemy. We barely glimpse the enemy. It's really about a survival story. I wanted to go through the experience with those characters. We were very, very clear that rather than using CG recreations of British destroyers, we were trying to find ship and birds that matched closely as possible rather than computer generate them. We would find the planes, the real planes, and fly them in real dogfights against each other and actually get the camera, get the actor up in the plane. We were going to do this for real as far as possible.
You have to go to the experts. We got a chap called Dan Freeman who owns like six spit fires and is a fantastic flier himself. We got them involved in the stage to talk about the real characteristics of the planes, how they flew, how they can fly, what G-forces the pilot can really sustain. When people do these dogfights using these computer generated planes, they inevitably violate the real laws of physics. We want to teach the audience how difficult this would be. How you bank chasing a plane and try to shoot it you have to get your gun sight ahead of it and anticipate how far it can move, what wind is going to do to the bullets and the tracer fire. Nothing crashed that wasn't supposed to. There was a rumor many months ago that I bought an antique plane and crashed it. We didn't do that. We built replicas.I think that for me the marine stuff was the most challenging. Even though this was by far the most complicated set of aerial scenes I've done, I'd done aerial work before on films like Dark Rises; I knew the pilots, I knew the cameraman, I knew how I would approach it, I knew how to split that work up. And I've done a lot of land-based action — not with a ton of extras, this was the biggest I've done — but I sort of worked my way up to it through the Dark Knight films and so forth. Boats, that was an entirely new thing for me. And very, very challenging.
I spoke to various filmmakers who'd shot in water before — spoke to Spielberg, spoke to Ron Howard about it, got some great advice. Both Steve and Ron very clearly felt that the best camera mode for shooting on a boat is handheld — even though we were shooting IMAX, because the camera man could steady themselves against the movement of the boat. That really proved to be the case. That's the way you get it done. It was very important for me to talk to actors before they read the script, which was very short — 75 pages, 76 pages, the shortest script I've ever written. Half a normal script. Very little dialogue, no back story. Just hints. So for example, when I went to talk to Mark Rylance about it, it was very important for him to understand the boat and to feel the tiller. He needed to feel the boat, to find how the physics of the situation could inform our understanding of the humanity of the character. The younger actors got very excited by that idea. It was vital for them, being there on the beach, being there out in the water. They're just really being in the elements and experiencing it and moving through it as people would have at the time.
We knew the water was going to be a huge component of what the actors were going to have to go through. They were gonna have to be in the water, out in the open, in the Channel — not for individual shots, but for the whole shoot — so it was very important that they be trained to deal with that safely. Our stunt guys put together a team of instructors. They did a lot of intense physical training for weeks where they would run in the waves, swim in the waves, get used to being in rough hazardous conditions. I think it was a shock to some of them, what was going to be required. The first shooting day was in some of the worst weather — very few film crews would gave carried on shooting. But for us, it looked marvelous with all this amazing foam washing up the beach. I’m known in the film business for having good luck with the weather. That's actually inaccurate. I often have terrible luck with the weather, but my philosophy is to shoot no matter what the weather is until the safety officer shuts us down. We tried to be opportunistic with how we shot. Grab the bad weather scenes when the weather's really bad, but always shooting, just keeping going, keeping going, no matter what the conditions are, as long as it's safe.
My cameraman Hoyte van Hoytema and myself put wetsuits or drysuits on; he had housings made for the cameras so they could go out in the waves; when it came to open water work, the camera could actually float out on the water — half in, half out. We're in there, swimming with them. I firmly believe in leading from the front. The fact that we were able to be out there with them and a part of the same physical elements they were dealing with, to some extent, experiencing what they're experiencing, was very much the spirit of those scenes. Being in it together and not sitting in a tent looking at video, I think it's vital for this kind of film. By the end of the film the idea behind Dunkirk that we're trying to get across to the audience is, it's not about individual heroics. It's about communal heroism. It's about the tremendous sense of community that was vital to the success of the operation. That's what makes the unique story and that's why I think it's always served as something of a rallying point for British people. I also think it's a very Universal story. It's really about the individual drive for survival. And the very universal concept of a desperation to get home.' — Telegraph