You might remember how flammable nitrate film was in the movie Inglourious Basterds. The story took a lot of liberties with history, but the Nazis actually did try to do away with nitrate film, also called celluloid, before World War II was over. Even today, Germany is working to rid itself of celluloid movies, which was the medium of choice for filmmakers up until 1951.
Celluloid is also extremely dangerous. It is essentially a solid form of nitroglycerin dragged across superhot carbon rods at extremely high speed. If celluloid combusts, which it can do at "car parked in the sun" temperatures, the fire generates its own oxygen, creating a flame which cannot be extinguished. It can burn underwater. It can burn beneath a fire blanket. It burns until the celluloid is gone, and any attempt to smother it creates clouds of poison gas.
That doesn’t sound like something you’d want to have in a crowded theater, but it was the reason why theaters were crowded. To ensure safety, celluloid film was segregated from the audience by the projection room, which was designed specifically to mitigate the danger of film fire. Read about nitrate film and the way theaters had to treat it at Atlas Obscura.
(Image credit: Library of Congress)