Actor Allusion: The character played by Janet Leigh gets in a car that is similar to one she drove in Psycho, with a piece of film's score playing in the background. She also tells Laurie that the shower is clogged - a reference to her character's famous death in the film. The character was originally intended as an Actor Allusion to PJ Soles - as it was named after her character from Carrie (1976), but it was played by Leigh after Soles failed to give them an answer, meaning that its name being similar to Psycho's antagonist Norman Bates was a coincidence. Likewise, Norma's motherly attitude towards Laurie was intended as this, to serve as a nod to the fact that mother and daughter were starring alongside each other.
Just about with Jodi Lyn O'Keefe and Josh Hartnett, who were both nineteen.
Averted with the other teens. Michelle Williams was seventeen, like her character Molly. Adam Hann-Byrd was actually an inversion - fifteen playing Charlie at seventeen.
Deleted Role: Charles S Dutton was cast as a detective, but his character was completely removed.
Executive Meddling: Moustapha Akkad wanted Michael Myers to survive so he could produce more Halloween films. Bob Weinstein however wanted Michael to die. The ending where Michael is apparently killed but revealed to have switched places with a paramedic in the next film was written as a compromise. It was apparently Bob Weinstein's idea to have Laurie decapitate Michael.
Follow the Leader: Much of the film is heavily influenced by Scream (1996), which had revived the slasher genre just over a year before this film came out. Dimension Films (who produced both movies), got Kevin Williamson (writer of Scream) on board to produce and write this film. Even it's original musical score from John Ottman was largely scrapped in favor of recycled music cues from Marco Beltrami's score for Scream. Both films largely follow the same formula with an "opening victim," snarky references to other slasher films, cameos, and Michael Myers was even supposed to be a killer-in-disguise in the original script (see below).
The fourth, fifth and sixth films were originally completely acknowledged; in fact the story of Laurie faking her death and assuming a new identity was written in order to accommodate her apparent death in the fourth film. This was clearer in a scene where a student in Laurie's class would give a report on "the Haddonfield murders" and Jamie's eventual death - and a heart-broken Laurie would excuse herself to throw up in the bathroom. This was cut so they could focus on Laurie's part of the franchise. However there are still nods to the other films - such as a picture of bloody scissors, referencing Rachel's death in the fifth film.
Jamie Lee Curtis wanted Laurie's life to be in complete shambles at the beginning of the film. Kevin Williamson instead suggested that Laurie have "more to lose".
PJ Soles, who played Lynda in the first film, was the original choice to play Laurie's secretary. The character's name Norma Watson was intended as a Shout-Out to Soles's Star-Making Role in Carrie (1976). She dithered for a while about whether to take the role and they eventually asked Janet Leigh after Soles failed to give them an answer.
John Carpenter was originally considered as a director. But after disagreements about money, he walked off the project.
The killer's death had multiple original possibilities - including Laurie trapping him in between a retractable gym floor and then stabbing him with a javelin - or being eventually cut in half by a helicopter's rotor blades.
The killer in this was intended to not be Michael Myers at all, but a copycat killer (and this would be explained in the next film). Ultimately Halloween: Resurrection explained that Myers had switched places with a paramedic last-minute instead.
The role filled by Nancy Stephens - reprising her part as Marion Whittington (nee Chambers) from the first two films - was originally for a new character. It would have been the daughter of Dr Loomis, Rachel, whose house is broken into.
Likewise a subplot featuring two detectives tracking Michael for Marion's murder was cut from the film. As such they only appear in one scene and are promptly forgotten.
One draft of the ending scene had Michael say Laurie's name to her face moments before she kills him. This would've been the very first and possibly only time we actually hear Michael speak in the entire franchise up to that point.
Another early draft had Ronnie as a woman called Hattie. She would die, as would Molly, who lives in the finished film.