Christopher Nolan let James Remar improvise one of the most shocking lines in ‘Oppenheimer’.

The moment comes in a scene where Cillian Murphy’s J Robert Oppenheimer meets with the US Secretary of War Henry Stimson and other government officials about where to drop the atomic bombs in Japan.

Stimson – played by James, 69 – tells the group to avoid using Oppenheimer’s atomic bomb to strike Kyoto as that’s where he and his wife honeymooned.

Christopher, 53, told the New York Times: “There’s a moment where James Remar… he kept talking to me about how he learned that Stimson and his wife had honeymooned in Kyoto.

“That was one of the reasons that Stimson took Kyoto off the list to be bombed. “I had him crossing the city off the list because of its cultural significance, but I’m like, ‘Just add that.’

“It’s a fantastically exciting moment where no one in the room knows how to react.”

Revealing how each character in the film did a he amount of research besides reading the Oppenheimer biography ‘American Prometheus’, Christopher added: “Each actor was coming to the table with research about what their real-life counterpart had been.

“They had tons of homework to do. They had a great resource with ‘American Prometheus’.

“They then did their own research and what it meant for me, which isn’t something I’d ever really been able to do in the past.

“So, for example, with the scene in the section classroom with all the scientists, we would be able to improvise the discussion.

“The script is there, but they could come into it with passion and knowledge based on all of their own learning.”

“Oppenheimer” has become a box office powerhouse for Universal Pictures since its 21 July release date, crossing the $180 million mark at the domestic box office in less than two weeks.

It has also soared past $412 million worldwide, with critics saying the numbers are “staggering” for a three-hour, R-rated biographical drama.