Robert Downey Jr. sang Cillian Murphy‘s praises in a recent interview with People. The Iron Man actor detailed the lengths Murphy went to in order to portray J. Robert Oppenheimer, the physicist known as “the father of the atomic bomb,” in the new movie Oppenheimer.

“I have never witnessed a greater sacrifice by a lead actor in my career,” Downey Jr. said. “He knew it was going to be a behemoth ask when Chris called him,” he added, referring to director Christopher Nolan. “But I think he also had the humility that is required to survive playing a role like this.”

The Avengers actor recalled Murphy declining invitations to spend time with the cast in order to prepare for the role. “We’d be like, ‘Hey, we got a three-day weekend. Maybe we’ll go antiquing in Santa Fe. What are you going to do?’ ‘Oh, I have to learn 30,000 words of Dutch. Have a nice time."”

Downey Jr. added, “But that’s the nature of the ask.”