Dawson Casting: Judd Nelson was 24 at the time, and Ally Sheedy and Emilio Estevez were both 22. Averted with Molly Ringwald and Anthony Michael Hall, both 16 at the time.
Enforced Method Acting: Judd Nelson remained in-character between shooting, and frequently taunted Molly Ringwald to the point of tears. John Hughes was on the verge of firing him, when Paul Gleason, ironically playing Principal Vernon, intervened on his behalf.
The original film's running time was close to two and a half hours long, thanks to a lot of improvised scenes by the cast. The studio said no audience would sit through a teen movie of that length and ordered trims. A few small scenes have surfaced in the trailers and the TV version to replace some of the censored footage, but most has never been released. Major edits included a different cut of the lunch scene and the club imagining more and more bizarre items appearing in the library until they all disappear when Vernon walks in. There were also several more scenes with the janitor, including his predictions for the club, like "a boob job and two kids" for Claire and Bender eventually committing suicide.
Real Life Relatives: Anthony Michael Hall’s sister Mary Christian and mother Mercedes Hall plays his character’s Brian Johnson’s sister and mother.
Throw It In: John Hughes told Judd Nelson to make up something off the top of his head for when he was in the air vent. Naked blonde walks into a bar...
What Could Have Been: Ally Sheedy and Molly Ringwald actually auditioned for the others' role. Emilio Estevez was originally cast as Bender, but when no suitable actor was found for the part of Andy, he agreed to take that instead, and let Judd Nelson play Bender instead
In addition, Rick Moranis was originally cast as Carl, but he left due to creative differences and was replaced by John Kapelos.
Hughes considered a sequel detailing what would happen upon the club's return to school the following Monday, but realized that it would be too complex a story to tell in film, and would be a logistical nightmare with regards to the actors reprising their roles. He danced around the idea of a novel, but ultimately never got around to it. He also tossed around the idea of doing a sequel every ten years, showing where the club was at in their lives, but for the above reasons (and possibly because of the Downer Ending predicted for Bender in a deleted scene) he decided against it. It's a double shame when you consider that all five of the actors said they would have reprised their roles in a heartbeat.