Cillian Murphy went through a "constant learning process" during the making of 'Oppenheimer'.

The 47-year-old actor stars as the titular character J. Robert Oppenheimer, the man behind the atomic bomb, in Christopher Nolan's much-hyped blockbuster and had to work hard to get under the skin of his alter ego.

Speaking to Digital Spy, Cillian said: "It's a constant learning process, a constant unravelling. You're like a detective the whole time.

"Then you just kind of pass it over to Chris (Nolan) and he puts it together. But (Oppenheimer) was a particularly unknowable character, particularly kind of opaque, morally and everything.

"Chris always used this expression with me from the beginning, which was that he was kind of dancing between raindrops."

Cillian has collaborated with the director for almost 20 years but was taken aback by the scale of his part in 'Oppenheimer'.

The '28 Days Later' actor explained: "I've said yes to him from the beginning and will continue to say yes.

"But I didn't know it was going to be this huge, ginormous, terrifying part. When it's Chris, he's a genius, and I don't throw that word around, but I think it definitely qualifies.

"I love re-collaboration. It's the richest thing when you're in work because you develop the shorthand and this trust and this respect and shared taste. So I hope that myself and Chris have developed that over the years and it just means you go straight to the work, direct line to the work. There's none of the fannying around."

Read the full interviews with the 'Oppenheimer' cast on the Digital Spy website at https://www.digitalspy.com/movies/a44543570/oppenheimer-cillian-murphy/