Cillian Murphy says Robert Downey Jr. is the most unpredictably brilliant actor he's ever worked with

While Cillian Murphy spent almost two months working alongside Robert Downey Jr., Emily Blunt, Matt Damon and Florence Pugh on the set of Oppenheimer, the other actors hardly ever saw the lead star off set.

Speaking to People magazine, Blunt said the set had a 'summer camp' feel about it.

“We were all in the same hotel in the middle of the New Mexican desert,” she said.

“We only had each other. Me and Matt [Damon] were roommates and we were like, ‘Let’s go to have dinner.'”

Blunt said Murphy never attended the dinners because of 'the sheer volume of what he had to take on and shoulder is so monumental'.

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Damon added: “Of course he didn’t want to come and have dinner with us. He couldn’t. His brain was just too full.”

Speaking to GQ this week, Murphy explained that when he's working, he just gets consumed by the role.

"That's just the way I work. I get very consumed by the work, and I don't really have time for hanging out. And in this movie, I was [regularly] skipping dinner, you know, so I wasn't great craic to hang out with. But that's just the way it was. It's just the nature of the work," the 47-year-old said.

However, while they didn't get to hang out, Murphy was blown away by Downey Jr.'s energy on set.

"But then, a lot of the scenes I have with Downey, it was quite loose and quite improvisational. I mean, acting with him was just extraordinary. He's just electrifying, the most available engaged, present, unpredictably brilliant actor I've ever worked with," he told GQ.

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Downey Jr. was similarly in awe of Murphy's performance and dedication to the role.

“I have never witnessed a greater sacrifice by a lead actor in my career,” he told People.

He added: “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.”

Pugh, who played Murphy's onscreen lover agreed, saying director Christopher Nolan 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," she told People.

"Working with him was hugely impressive," she continued. "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."

Oppenheimer is currently screening in cinemas around the country.