Oppenheimer: Cillian Murphy on "Mind-Blowingly Good" Work With Robert Downey Jr. (Exclusive)

As far as Christopher Nolan films go, Oppenheimer has one of the auteur's most expansive ensembles yet. In the lead role of J. Robert Oppenheimer is Cillian Murphy, the Peaky Blinders alumnus that find himself flanked by Emily Blunt, Matt Damon, and Robert Downey Jr. amongst many others. In fact, the latter churns out a "mind-blowingly" good performance as late Atomic Energy Commission chairman Lewis Strauss.

"I think Robert Downey Jr. is just absolutely electrifying in that role. I think he's mind-blowingly good in it. We talked a lot, me and Robert and Chris [Nolan] about it, about this dynamic between that relationship because it's really, really interesting and complex," Murphy tells us. "When you think that one man's motivation in destroying another man can be about a slight at this tiny, tiny event, so he feels slighted. It was just fascinating to talk about that."

In real life, Strauss and Oppenheimer had a contentious relationship with the former distrusting Oppenheimer because of his alleged Communist ties. At one point, Strauss even lobbied then-President Dwight Eisenhower to revoke Oppenheimer's security clearance.

"Then the scenes where we did get to work together, it was just the best," Murphy adds. "He's the most wonderful scene partner and so generous, so focused, but you never know what you're going to get in a brilliant way. Some of those scenes, Chris let us loose a little bit so we could actually extemporize and improvise a little bit. They were some of my favorite scenes in the movie."

Joining Murphy, Downey, Blunt, and Damon are Florence Pugh as Jean Tatlock, Benny Safdie as Edward Teller, Michael Angarano as Robert Serber, Josh Hartnett as Ernest Lawrence, Dylan Arnold as Frank Oppenheimer, David Krumholtz as Isidor Isaac Rabi, Matthew Modine as Vannevar Bush, Josh Peck as Kenneth Bainbridge, Devon Bostick as Seth Neddermeyer, Matthias Schweighöfer as Werner Heisenberg, Christopher Denham as Klaus Fuchs, Guy Burnet as George Eltenton, Danny Deferrari as Enrico Fermi, Emma Dumont as Jackie Oppenheimer, Gustaf Skarsgård as Hans Bethe, Trond Fausa Aurvåg as George Kistiakowsky, and Gary Oldman as Harry S. Truman.

Opening on July 21st, Oppenheimer is based on Kai Bird and Martin J. Sherwin's Pulitzer Prize-winning novel American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer.