A number of famous Austrian and German actors appear in small parts, including Paul Hörbiger as the Hausmeister (janitor). Dr. Winkel is played by Erich Ponto, who played Peachum in the original production of the Threepenny Opera, and Baron Kurtz by Ernst Deutsch, the Rabbi's assistant from the 1920 movie Der Golem, wie er in die Welt kam.
Money, Dear Boy: Orson Welles took the role as Harry Lime in order to fund his film adaptation of Othello. Also crosses over into What Might Have Been, in that Welles was offered either a straight paycheck or a third of the film's gross. He took the paycheck (being desperate for cash), but considering how huge a hit the movie was, had he taken the percentage, he might never have had to do paycheck work again.
Reality Subtext: Graham Greene used to work for British Intelligence in WW2, where his boss and friend was the Double Agent Kim Philby. It's been speculated that Lime (a charismatic yet amoral friend secretly working for Soviet Intelligence) reflected underlying suspicions Graham had of Philby. In a particular irony, after Guy Burgess and Donald Maclean fled to Moscow to escape arrest, Philby fell under suspicion of being "the third man" in their spy ring.
What Could Have Been: Cary Grant was the first actor considered to play Lime. For reasons unknown, he was passed over (apparently, though, he was still a frequent visitor to the London studio shooting sessions) in favor of Orson Welles over David O Selznick's strident objections. Similarly, James Stewart was first considered to play Holly Martins — here Selznick won, as he demanded Joseph Cotten, who was under contract to Selznik at the time.