Inception and Interstellar director Christopher Nolan is a master of the large-scale blockbuster, but he still thinks the small things are the most important.

Nolan has revealed that he scheduled filming for Oppenheimer around lead actor Cillian Murphy so that the star would not have to wear a wig for it. In the movie, Murphy plays J. Robert Oppenheimer with various different hairstyles as Nolan tracks the life of the so-called "father of the atomic bomb" from the 1920s to the '60s.

"Anytime you get into myriad locations, a lot of different actors, it's always going to be a puzzle," Nolan told The New York Times in a new interview after being asked about shooting such an ambitious project. "I did insist on scheduling it around Cillian's haircut. [Laughs] Because I'm very allergic to wigs in movies. I really wanted the film to not have any obvious artifice when it came to the way characters presented themselves."

Murphy has been a stalwart supporting player in several of Nolan's previous movies, including 2005's Batman Begins and 2010's Inception. But the Irish actor was unaware that the director had even written a script about Oppenheimer until Nolan called him up and asked if he was interested in portraying the film's titular physicist.

"Chris' way of operating is that he just calls you out of the blue," Murphy said in EW's recent Around the Table for Oppenheimer. "I genuinely had no idea. He said he was making a movie about Oppenheimer and he said, 'I'd like you to play Oppenheimer.' I had to sit down. It was kind of overwhelming."

Nolan admitted in the same conversation that his habit of phoning Murphy without warning to offer him roles does have a downside. "[It's] a fun way to do it," said the director, "but it means that it's very difficult to call you to go out to dinner or something. Because every time you answer the phone it's like, 'What's it going to be?'"

Oppenheimer hits theaters July 21. Watch the full Around the Table video — featuring Nolan, Murphy, Matt Damon, Emily Blunt, and Robert Downey Jr. — above.

EW's Around the Table for Oppenheimer was conducted before the start of the SAG-AFTRA strike.

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