 The scene where Superman leaves Lois on her balcony and then Clark  is heard knocking on her door was filmed in one continuous take.  Christopher Reeve (qv) had to leave the scene as Superman and  enter the door of the apartment as Clark Kent, which was physically  impossible in such a short amount of time. To accomplish the shot,  the footage of Superman flying away was filmed first and then played  back with a projector. When Lois waves goodbye to Superman, shes  waving goodbye to a movie screen. She then answers the door to a  "live" Reeve dressed as Clark Kent.