It wasn’t! I was very gratified by the response to the film and hugely relieved. I took a lot of risks and did a lot of things that were outside my comfort zone with “Dunkirk.” I removed a lot of my safety nets, particularly in the screenwriting area where I’d become, I think, fairly proficient in the use of dialogue to express back story and character. And when I wrote this screenplay, I decided to remove all that and look at it in a different way. So I felt like I challenged myself and really tinkered with the fundamentals of large-scale filmmaking. And I was certainly nervous that the film wouldn’t work in the marketplace.
Source : http://www.latimes.com/entertainment/envelope/la-en-mn-christopher-nolan-20180104-htmlstory.html