Cillian Murphy Underwent Concerning Diet Regimen for 'Oppenheimer'

The "Peaky Blinders" star did not eat much while he was filming "Oppenheimer."

Cillian Murphy went to great lengths for his performance in Oppenheimer, with some of his co-stars recently indicating he had a concerning diet regimen for the role. Speaking to ET ahead of the film's premiere, Matt Damon said, "We invited Cillian to dinner every night and he never went," recalled Matt Damon. Emily Blunt added that Murphy was eating, "like, one almond, I think, most nights, or like, a little slice of apple."

Damon then added, "He was losing so much weight for the part that he just didn't eat dinner ever." Explaining his dieting choices, Murphy quipped, "They were hanging out. I was in the bath, learning lines." Damon recalled, "We had dinner every night," with Blunt adding, "So many margaritas." Damon then aded, "It was like the dining hall, you'd go in and you'd see everybody and so everyone would just end up sitting together. It always helps when you're kind of immersed with the cast."

In Oppenheimer, Murphy stars as J. Robert Oppenheimer, the theoretical physicist who helped develop the first nuclear weapons. Damon portrays Lieutenant General Leslie Richard Groves Jr., the Army Corps of Engineers officer who was in charge of the Manhattan Project for the government. Blunt plays Oppenheimer's wife, Katherine "Kitty" Oppenheimer.

The new film is a biopic-thriller about the life and career of Oppenheimer. It is directed, written, and co-produced by Christopher Nolan, and is based on the 2005 biography American Prometheus by Kai Bird and Martin J. Sherwin. Murphy stars as Oppenheimer, with additional performances from a star-studded cast that includes Robert Downey Jr., Florence Pugh, Josh Hartnett, Casey Affleck, Rami Malek, Kenneth Branagh, Gary Oldman, Benny Safdie, and David Krumholtz, among many others.

Nolan spoke with ET as well, offering some insight into the true story behind the film. "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," Nolan explained. "A small possibility, [but they] couldn't eliminate it completely through theory, and yet they went ahead and they pushed that button. As a filmmaker, that's the kind of story you're looking to tell." Oppenheimer opens in theaters on Friday, July 21.