Like many who have seenOppenheimer,Robert Downey Jr.was floored with Cillian Murphy’s transformative performance as J. Robert Oppenheimer, the American physicist wholed the U.S. effort to create an atomic bombduring World War II.

“He knew it was going to be a behemoth ask when Chris called him. But I think he also had the humility that is required to survive playing a role like this,” says Downey.

Melinda Sue Gordon/Universal Pictures

Cillian Murphy is J. Robert Oppenheimer and Robert Downey Jr is Lewis Strauss in OPPENHEIMER, written, produced, and directed by Christopher Nolan.

“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.’ But that’s the nature of the ask.”

Universal Pictures

Florence Pugh is Jean Tatlock and Cillian Murphy is J. Robert Oppenheimer in OPPENHEIMER, written, produced, and directed by Christopher Nolan.

Florence Pugh, 27, who shares most of her scenes with Murphy as Jean Tatlock, one of the major loves of Oppenheimer’s life, offered further praise.

“Chris had one of the most incredible leads in Cillian,” she tells PEOPLE. “He is an actor that I have been watching for quite some time and have been desperate to work with for ages. You’d have to be mad to say no. It was truly one of the best experiences that I’ve had.”

“Working with him was hugely impressive,” she continues. “Every single day he shows up knowing every single possible way, intonation, inflection of how to bring this character to life. That was hugely impressive to me. There’s a reason why he is one of the greats.”

Never miss a story — sign up forPEOPLE’s free daily newsletterto stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

Emily Blunt is Kitty Oppenheimer and Cillian Murphy is J. Robert Oppenheimer in OPPENHEIMER, written, produced, and directed by Christopher Nolan.

Murphy’s commitment to the role included following a very restrictive diet to closer resemble Oppenheimer.

His costarEmily Blunt, who plays his onscreen wife Kitty Oppenheimer, recently toldExtra, “He had such a monumental undertaking. And he could only eat, like, an almond every day. He was so emaciated.”

The Irish actor addressed his dramatic weight loss for the role during a recent interview withIndieWire, stressing, “I don’t advise it.”

Oppenheimeris now playing in theaters.

source: people.com