The scene in which all characters sit in a circle on the floor in the library and tell stories about why they were in detention was not scripted. Writer and director John Hughes told them all to ad-lib.
John Hughes later said that his biggest regret about this film was using the breaking glass effect during the marijuana scene.
Judd Nelson (John Bender) stayed in character off-camera, even bullying Molly Ringwald. John Hughes nearly fired him over this, but Paul Gleason (Richard Vernon) defended Nelson, saying that he was a good actor, and he was trying to get into character.
In 2010, Molly Ringwald (Claire Standish) and Anthony Michael Hall (Brian Johnson) told Vanity Fair magazine that John Hughes was receptive to actors' and actresses' improvisations, and some of them (including Brian's reason for having a fake ID, "so I can vote") made it into the final film.
John Hughes: [The Beatles] Principal Richard Vernon (Paul Gleason) is named after a minor actor from A Hard Day's Night (1964). When the principal asks the janitor what he wanted to be when he grew up, the janitor replies that he wanted to be John Lennon.