Apparently, the realism desired by Christopher Nolan for Oppenheimer was not limited to wanting to recreate the explosion of an atomic bomb without using digital effects . As he himself revealed, given the theme of the film and its implications, he wanted the extras on stage in the segments of the film set at Los Alamos to be real scientists.
The filmmaker explained that:
I thought that the question (realism, ed.) should also be applied to the extras. We were in the real Los Alamos and employed several scientists as extras. We needed the extras to have reactions and improvise and we got this kind of improvisation based on very polite speeches in response. Very fun to hear. You happen to be on set surrounded by extras who are there thinking about lunch. This time they were thinking about the geopolitical implications of nuclear weapons and they really knew a lot about it. It was a very good daily reminder: we had to be focused, adhere to the story and aware of what we were doing.
The film is based on the Pulitzer Prize-winning book American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer by Kai Bird and the late Martin J. Sherwin. The film is produced by Emma Thomas, Charles Roven of Atlas Entertainment and Christopher Nolan.
Oppenheimer is shot in both IMAX® 65mm and large format 65mm film which includes, for the first time ever, sections in black and white IMAX® analog photography.