19. The Keymaster Gag
The role of Louis Tully was originally written for John Candy which can be seen in the film's storyboards. When Candy wanted to alter the character to be an uptight German, the role instead went to Rick Moranis who was perfect to play the tiny, anxious nerd.
Ghostbusters has it's dry wit but it's not exactly known for subtlety. The sexual overtones of "gatekeeper" and "keymaster" are obvious, so much so that they can eclipse a great gag written into Louis' character.
Before he's even possessed by the Terror Dog Vinz Clortho, Louis gets locked out of his apartment on three different occasions. Probably not the kind of guy you'd want to possess to become "the keymaster"
It all works out in the end though. When Vinz makes his way back to the building, the door to Dana's apartment supernaturally swings open for him as he arrives.