Cillian Murphy takes center stage in Christopher Nolan‘s upcoming movie Oppenheimer, and the director is looking back on his casting decision.

The 47-year-old actor had already worked on five movies with Christopher before taking a lead role in what’s shaping up to be one of the summer’s biggest blockbusters.

During a recent interview, Christopher explained why Cillian was the obvious choice to portray the “father of the atomic bomb.”

Read more about Christopher Nolan’s decision to cast Cillian Murphy…

Christopher told The Hollywood Reporter that he instantly pictured Cillian embodying J. Robert Oppenheimer in the project.

“I’ve been staring at the cover of the book American Prometheus for so many months, and there’s this photograph, black and white, a light blue-eyed stare, very intense, of this guy,” he recalled. “And I thought, ‘Well, I know who could do that.’”

Cillian reflected on getting the offer: “He’s just so understated and matter-of-fact and British. It was like, ‘Cillian, look, I have a script, it’s Oppenheimer and I’d like you to play Oppenheimer.’ That was it.”

Oppenheimer premieres in theaters on July 21. If you missed it, the cast just made a big decision while attending a promotional event for the movie.

Press play on the Oppenheimer trailer below…