Filming for Christopher Nolan’s atomic bomb drama, Oppenheimer has begun. A film about the man who developed the most destructive weapon of World War II. Cillian Murphy will star as J. Robert Oppenheimer, the leader of the Manhattan Project. Oppenheimer will be the Nolan alum’s first leading role after playing supporting roles in five of the director’s films (three Dark Knight films, Inception, and Dunkirk).

When it came time to prepare for the role, Murphy decided to skip the mechanics of what makes an atomic bomb in favor of looking into the mind of the man who invented it.

“[I prepped by doing] an awful lot of reading,” Murphy told The Guardian. “I’m interested in the man, and what [inventing the atomic bomb] does to the individual. The mechanics of it, that’s not really for me — I don’t have the intellectual capability to understand them, but these contradictory characters are fascinating.”

Oppenheimer is based on Kai Bird and Martin Sherwin’s biography American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, which was on Murphy’s reading list.

As exciting as it may be to lead a Nolan tentpole for the first time, Murphy never cared about the size of the role when it came to Nolan.

“I’ll always turn up for Chris, whatever the size of the part,” Murphy said. “Chris will call me up, and I’m there. Isn’t it wonderful that filmmakers are still making challenging, demanding films within the studio system, shot on film? I think he’s flying the flag. Him, Paul Thomas Anderson, and Quentin Tarantino are fantastic filmmakers making interesting work on a massive scale.”

Nolan has wrangled a true stable of stars for Oppenheimer. Joining Murphy in the movie is Emily Blunt as biologist and botanist Katherine “Kitty” Oppenheimer, Matt Damon as Manhattan Project director Gen. Leslie Groves Jr., Florence Pugh as psychiatrist Jean Tatlock, and Robert Downey Jr. as Lewis Strauss, a founding commissioner of the U.S. Atomic Energy Commission.

The film also stars Rami Malek, Dane DeHaan, Dylan Arnold, David Krumholtz, Alden Ehrenreich, and Matthew Modine.

Oppenheimer will release in North American theaters on July 21, 2023.