Cillian Murphy says that being cast in 'Oppenheimer' was "one of the best days" of his life.

Cillian Murphy was delighted to land the lead role in 'Oppenheimer'

Cillian Murphy was delighted to land the lead role in 'Oppenheimer'

The 46-year-old actor is to star as J. Robert Oppenheimer – the 'father of the atomic bomb' – in the upcoming film and was ecstatic that director Christopher Nolan trusted him to play the lead role in the flick.

Speaking to Entertainment Weekly, Cillian said: "It was one of the best days of my life, I'll tell you that, when you called me.

"I'll always turn up for Chris, no matter what the part is, but, secretly, it's a dream to play the lead part. The thing was, I had no idea. There was no preamble or anything, I just got the call. So it was incredibly exciting, and daunting, and terrifying, all at the same time."

Christopher also has fond memories of breaking the news to the 'Peaky Blinders' star that he would be leading the project.

The 'Inception' director recalled: "No one knew what I was up to, no one knew what I was doing.

"To be able to pick up the phone, and call you, and be like, 'This is the one where you carry the movie and really get to show what you can do,' it's honestly one of my favourite moments in the movie business, when I had that conversation with you."

Cillian heads an all-star cast that includes Emily Blunt, Matt Damon and Florence Pugh and explained how he had to "raise his game" on a daily basis to work with his contemporaries on set.

He said: "Every day, you had these phenomenal actors, who are heroes of mine, coming in. Every day, you were having to raise your game to work with these legends.

"Everybody was so unbelievably well-prepared. Every single actor, no matter what size their role or the significance of their character in history, each one of them had this massive depth of knowledge that they could draw on."