Many of the actors who played the Nazis were in fact European Jews who had fled Nazi occupation.
Rick's Cafe was one of the few original sets built for the film, the rest were all recycled from other Warner Bros. productions due to wartime restrictions on building supplies.
During the scene in which the "La Marseillaise" is sung over the German song "Die Wacht am Rhein" ("The Watch on the Rhine"), many of the extras had real tears in their eyes as a large number were actual refugees from Nazi persecution in Germany and elsewhere in Europe and were overcome by the emotions the scene brought out. The scene was inspired by Jean Renoir's La gran ilusión (1937), in which French soldiers in a German POW camp sing the song as a similar gesture of defiance. Marcel Dalio, who played Emil the croupier, had also appeared in The Grand Illusion (as Lieutenant Rosenthal). "La Marseillaise" was written in 1792 by Claude Joseph Rouget de Lisle in Strasbourg after the declaration of war by France against Austria and was originally titled "Chant de guerre pour l'Armée du Rhin" ("War Song for the Rhine Army").
Because the film was made during WWII the production was not allowed to film at an airport after dark for security reasons. Instead, it used a sound stage with a small cardboard cutout airplane and forced perspective. To give the illusion that the plane was full-sized, they used little people to portray the crew preparing the plane for take-off. Years later the same technique was used in Alien, el octavo pasajero (1979), in the scene where the crew discovers the dead "space jockey," with director Ridley Scott's son and some of his friends in scaled-down spacesuits.
The first scene that Michael Curtiz and company shot was one of the flashback scenes in Paris, which caused some problems for both Humphrey Bogart - because, in his own words, "I'm not up on this love stuff and don't know just what to do" - and for Ingrid Bergman - because, as the script had not yet been finished, she didn't know whether her character was supposed to be in love with Rick or Victor Laszlo. Curtiz, who did not know himself, told her to "play it in between."