Cillian Murphy, playing Oppenheimer, finally gets to lead a Christopher Nolan film

The day Christopher Nolan called Cillian Murphy about his new film, Oppenheimer, Murphy hung up the phone in disbelief.

The Irish actor, though a regular presence in Nolan films going back almost two decades, had always been a supporting player. This time, Nolan wanted him to lead.

“He’s so understated and self-deprecating and, in his very English manner, just said, ‘Listen, I’ve written this script, it’s about Oppenheimer. I’d like you to be my Oppenheimer,’” Murphy, 47, said earlier this year. “It was a great day.”

For Murphy, it is never not exciting to get a call from Nolan. It’s just hard to predict if he’s going to. He knows there are some movies he’s right for and some movies he isn’t.

“I have always said publicly and privately, to Chris, that if I’m available and you want me to be in a movie, I’m there. I don’t really care about the size of the part,” he said. “But deep down, secretly, I was desperate to play a lead for him.”

Murphy first met Nolan in 2003. He was brought in to screen test for Batman — not just the movie, the character. Murphy knew he wasn’t right for the Dark Knight, but he wanted to meet the man who’d directed Insomnia and Memento. They hit it off and Murphy got to tap into a sinister intensity to play the corrupt psychiatrist Dr. Crane/Scarecrow, who would go on to appear in all three films. Nolan would also call on Murphy to be the conflicted heir to a business empire in Inception and a traumatized soldier in Dunkirk.

“We have this long-standing understanding and trust and shorthand and respect,” Murphy said. “It felt like the right time to take on a bigger responsibility. And it just so happened that it was a f---ing huge one.”

Soon after the phone call, Nolan flew to Dublin to meet Murphy and hand him a physical copy of the script, which he devoured right there in Nolan’s hotel room. It was, he said, the best he’d ever read.

Then the scale of it started to sink in.

This would be a film about the charismatic and controversial theoretical physicist who helped create the atomic bomb. Oppenheimer and his peers at Los Alamos would test it on July 16, 1945, not knowing what was going to happen. There was a non-zero chance that the heat from the explosion could set off a chain reaction that would ignite the atmosphere and literally set the world on fire.

It didn’t, but several weeks later the United States would drop those bombs on the Japanese cities of Hiroshima and Nagasaki, killing tens of thousands of people and leaving many with lifelong injuries. Soon, the United States was at work to strengthen its nuclear arsenal, developing plans to work on an even more catastrophic weapon: the hydrogen bomb.

As Nolan has said, “Like it or not J. Robert Oppenheimer is the most important person who ever lived.”

Oppenheimer, which opens in theatres July 20 in the UAE, features a starry cast including Emily Blunt as Oppenheimer’s wife Kitty, Matt Damon as the man who hired Oppenheimer for the job at Los Alamos, Robert Downey Jr. as a founder of the Atomic Energy Commission and many more rounding out the pivotal players in and around this tense moment in history.

“You realize this is a huge responsibility. He was complicated and contradictory and so iconic,” Murphy said. “But you know you’re with one of the great directors of all time. I felt confident going into it with Chris. He’s had a profound impact on my life, creatively and professionally. He’s offered me very interesting roles over and I’ve found all of them really challenging. And I just love being on his sets.”

Murphy continued: “Any actor would want to be on a Chris Nolan set, just to see how it works and to witness his command of the language of film and the mechanics of film and how he’s able to use that broad canvas within the mainstream studio system to make these very challenging human stories.”

Over the years, Murphy has come to appreciate that with Nolan there’s always something deeper to discover than what’s literally on the page. Dunkirk, he recalled, was only 70 pages and there wasn’t much to his character, not even a name.

• Hollywood actors' strike: Cillian Murphy, Robert Downey Jr’s ‘Oppenheimer’ New York red carpet cancelled