A French postman fell off of his bicycle on the Champs Elysées when he saw some extras dressed as German soldiers crossing the avenue during a lunch break. He ran out screaming, "They're back! They're back!"
One of the main reasons for the movie being filmed in black and white: the French authorities refused to allow red and black Nazi flags to fly in Paris, even for a movie. They agreed only to the use of black and gray Nazi flags.
Several French civilian vehicles are accurately shown with cylindrical tanks on their roofs. These tanks contained compressed natural gas. During World War II, over 60,000 French vehicles were converted to natural and wood gas fuel.
This film was completely shot in Paris. One hundred eighty filming locations were used within the city and its surrounding areas.
Orson Welles, who had a role in the movie, was arrogant with director René Clément and refused to directly talk with him, although Welles spoke fluent French. He used Yves Boisset as a translator. According to Boisset, Welles was jealous that Clément had been given a mammoth-budget movie to direct.