Christopher Nolan's projects are known to be shrouded in secrecy. 

When casting for Batman Begins, the director famously turned up to Michael Caine's house on a Sunday morning with the script and refused to leave until Caine had read it so he could take it home with him. Caine ended up serving Nolan lunch while he read it.

Twelve years on, Nolan is one of the most high profile directors in the world, and such he and his team have to go to extreme lengths to maintain the same levels of secrecy. Nolan was reportedly so guarded with the script for Dunkirk that he allowed very few crew members to read it. 

IndieWire estimates that of the 600 or so people that contributed to bringing the film to the big screen, only about twenty of those crew members were allowed to read the script during production. The twenty included the director of photography, production designer, first assistant director and property master.

The film's Art Director, Stephane Cressend said that this meant that there "was a lot of guessing,”

“It’s not harder, it’s just a different way to work,” he said. “You have to get accustomed to a new way of seeing the movie.”

The secrecy went well beyond just keeping the script closely guarded. There were reportedly no monitors on set aside from a small handheld one that the director himself viewed footage on. Meaning Nolan was the only one to see the footage that was being recorded. During his interview with us, Barry Keoghan confirmed that Nolan uses just one handheld monitor and stands right beside the camera when filming. 

There was also a ban on taking photos on set. The only crew members allowed to take photos were the on set dresser and the script supervisor for continuity purposes. A member of the costume department reportedly almost got fired after they took a photo of a soldier's outfit for continuity. 

The cast weren't immune to the secrecy either. While they all received their own personal hard copies of the script, they also had to go to extreme lengths to retain secrecy. Kenneth Brannagh revealed that he personally had to hand his copy back to Nolan for revisions. 

“I had to fly to Los Angeles and hand it directly back to him,” Branagh he told Variety. “I felt like I was on a spy film.”

Via IndieWire