Robert Downey Jr. praises Cillian Murphy in Oppenheimer: Greatest Sacrifice.

Cillian Murphy receives some of the best reviews of his career for starring in Christopher Nolan’s Oppenheimer, but no praise could be greater than this paean from co-star Robert Downey Jr., “I’ve never seen a greater sacrifice from a lead actor in my career,” the Iron Man star told People magazine of Murphy’s performance as J. Robert Oppenheimer.

“When Chris called him, he knew it was going to be a huge question,” Downey Jr. added. “But I think he also had the humility it takes to survive in a role like this. We said, “Hey, we have a three-day weekend.” Maybe we’ll go antique shopping in Santa Fe. What are you going to do?’ ‘Oh, I have 30,000 words of Dutch to learn.’ Have fun.’ But that is in the nature of the question.”

Oppenheimer was shot in 57 days, one of Nolan’s fastest shoots, and Murphy is the focus of almost every scene. The cast and crew lived together in the same hotel during the film’s production, but Murphy never came to dinner with his castmates due to the intensity of the lead role.

“Obviously he didn’t want to have dinner with us,” Matt Damon previously told People magazine. “He could not. His brain was just too full.”

Emily Blunt argued that Murphy didn’t attend cast dinners because “the sheer volume of what he had to handle and shoulder is so monumental.”

For Murphy, “Oppenheimer” marks the biggest leading role of his film career to date. This pressure isolated the actor. “You know you feel like these big roles and these responsibilities are kind of overwhelming,” he told People.

Murphy also lost weight for the role, though he didn’t reveal how much. In the film, the actor plays the role of theoretical physicist J. Robert Oppenheimer, who is hired to lead the Manhattan Project in Los Alamos, New Mexico to develop the atomic bomb. Downey Jr. plays Lewis Strauss, chairman of the Atomic Energy Commission who later pursued Oppenheimer as a Soviet spy.

“Chris had one of the most incredible leads with Cillian,” Florence Pugh added to People. “He’s an actor that I’ve been following for quite some time and have really wanted to work with for ages. You’d have to be crazy to say no. It really was one of the best experiences I’ve ever had.”

“Working with him was extremely impressive,” she added. “Every day he shows up and knows every single way, every inflection, every inflection to bring this character to life. That impressed me enormously. There’s a reason he’s one of the greats.”

“Oppenheimer” is now in theaters nationwide from Universal Pictures.