Unemployment and inflation were so bad in Germany at the time that the producers had no trouble finding 500 children to film the flooding sequences.
Much to Fritz Lang's dismay, Adolf Hitler and Joseph Goebbels were big fans of the film. Goebbels met with Lang and told him that he could be made an honorary Aryan despite his Jewish background. Goebbels told him "Mr. Lang, we decide who is Jewish and who is not." Lang left for Paris that very night.
Fritz Lang wanted 4,000 bald extras for the Tower of Babel sequence, but Erich Pommer could only find 1,000 willing to shave their heads. Since the scene was shot in the spring, these extras got to swelter under the hot sun shooting the exteriors as they hauled prop rocks and real tree trunks across the landscape. Some got sunburns on their scalps from the lengthy shoot. After shooting, Lang ordered the shot run through the optical multiplier to make the 1,000 extras seem like the 4,000 he had originally wanted.
The multiple-exposed sequences were not created in a lab but right during the filming on the set. The film was rewound in the camera and then exposed again right away. This was done up to 30 times.
No optical printing system existed at the time, so to create a matte effect, a large mirror was placed at an angle to reflect a piece of artwork while live footage was projected onto the reverse. To expose the projected footage, the silvering on the back of the mirror had to be scraped off in strategically appropriate places. One mistake would ruin the whole mirror. This was done for each separate shot that had to be composited in this manner. This procedure was developed by Eugen Schüfftan and is known as the "Schufftan Process."