'Oppenheimer': Christopher Nolan and Cillian Murphy on the Film's 'Dramatic and Important' Story (Exclusive)

Christopher Nolan has been ready to tell the story of J. Robert Oppenheimer, the "father of the atomic bomb," for years. And he's doing just that in his latest epic, Oppenheimer.

"A lot of people are familiar to some degree with Oppenheimer -- that he was involved in the Manhattan Project, he ran the laboratory in Los Alamos during World War II, that they were in a desperate race against the Nazis to be the first to harness this power," the director explained to ET's Rachel Smith. "But what happened to him afterwards is equally dramatic and important and it's not so well known. The film has a lot to do with the consequences of actions -- sort of going ahead and doing these things and then having to deal with the consequences for years afterwards."

"We live in the world that Oppenheimer created," he added. "It's a pretty fascinating set of events. I just don't know of a story as paradoxical, just as full of impossible situations."

The director was so fascinated by the people and events of the Manhattan Project, in fact, that he actually dropped an Easter egg about Oppenheimer in his previous film, Tenet.

"There's a reference to this idea that when Oppenheimer was running the Manhattan Project and they were doing their calculations, early on they saw the possibility that when they triggered the first atomic device, to test it, they might start a chain reaction that set fire to the atmosphere and destroyed the whole world."

"A small possibility, [but they] couldn't eliminate it completely through theory, and yet they went ahead and they pushed that button," he continued. "And I just thought, if you could go there with the audience, if you could take the audience into that room, where they're having those conversations, where they're making those decisions and ultimately pushing that button, that's as dramatic a scene I could imagine in motion pictures. So, as a filmmaker, that's the kind of story you're looking to tell."

Nolan found his Oppenheimer in Irish actor Cillian Murphy, who called the opportunity to play the theoretical physicist "a gift, a total gift."

Notoriously secretive about his scripts, the director actually flew to meet with each of his prospective stars in person, to allow them to read his physical copy of the Oppenheimer script.

"The extraordinary thing about the script was it's written in the first person, which is the first time I'd ever encountered that," Murphy shared of his first impression. "You realize then this is something different. They made this very clever thing in the movie -- the Oppenheimer sequences are in color, and then the Strauss sequences are in black and white. So it's kind of subjective-objective, and that's just amazing."

Lewis Strauss, the American businessman and naval officer who later became chairman of the the U.S. Atomic Energy Commission (AEC) -- and ultimately headed up the hearings that revoked Oppenheimer of his security clearance -- is played in the film by Robert Downey Jr., who told ET he was thrilled to count himself among the A-list cast, which also includes Matt Damon, Emily Blunt, Florence Pugh, Kenneth Branagh and more.

"You have pretty much a who's who of several generations of actors," he noted. "And we all know we're doing something with a master filmmaker. We're all there for the love of it, because this was a money gig for none of us. It was an exceptionally taxing venture. And we'd all do it again."

Downey referred to Murphy's performance in the film as a "heroic effort," but his co-star humbly attributed his portrayal to the "safe environment" Nolan created for his actors on set.

"He creates this really kind of like a laboratory, where you can just try stuff out and he just lets you off," he explained. "And the other thing is, I was thinking about it, no scene ever gets left behind, do you know what I mean? No scene is less than another."

Certainly no stranger to star-studded ensembles, particularly throughout his time in the MCU, Downey received praise of his own from Nolan -- who marveled at the actor's talent, some of which has been hidden away behind an iron suit for the last decade or so.

"Robert Downey Jr. is somebody who I've wanted to work with forever," the director shared. "He's an extraordinary movie star with this amazing charisma, you know, Iron Man, Sherlock Holmes, these great films. But the thing that people sometimes lose in that is he's also one of the greatest actors."

"To meet with him and show him the script and say, what about just inhabiting this real-life figure? Losing any associations we have with Robert Downey Jr., seeing [you] do something that you've never done before," he recalled. "I think he was thrilled to be offered the challenge like that and I think his work in the film is really going to surprise people. I think it's quite remarkable."

Blunt and Damon likened the vibe on the film's New Mexico set to "summer camp," and while Murphy admitted that he was often off on his own "learning lines," he had nothing but praise for his castmates and their work on the world-changing epic.

"I liked working with Florence Pugh, [she] was amazing," Murphy raved. "She's something else. And I've admired her work for a long time, so that was a real treat to get to work with her."

"Everyone smashed it. Like, everyone's unbelievable," he added. "'Cause Chris expects excellence, that's just a given, and everyone turned up and delivered. Every single actor."

Oppenheimer is in theaters July 21.

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