Robert Downey Jr has praised Cillian Murphy's "sacrifice" to play J. Robert Oppenheimer.

The 47-year-old actor plays the theoretical physicist, who is known as the "father of the atomic bomb", in Christopher Nolan’s 'Oppenheimer' and co-star Downey Jr was impressed with Murphy's dedication to the role.

Robert, 58, who plays Atomic Energy Commission chairman Lewis Strauss in the film, told PEOPLE: "I have never witnessed a greater sacrifice by a lead actor in my career. 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.

"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."

And, Florence Pugh, who plays Jean Tatlock, one of the major loves of Oppenheimer's life in the movie, was also impressed with the actor, calling working with him "one of the best experiences that I've had".

She said: "Chris had one of the most incredible leads in Cillian. 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. 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."