John Candy was originally supposed to play Louis Tully, but he backed out after his idea for Louis to have a German accent and two pet schnauzers was rejected. (Source.)

When the ecto-containment unit blows, famous porn star Ron Jeremy can be seen in the crowd. (Source.)

Slimer's blobbiness was inspired by the body shape of Aykroyd's best friend John Belushi, who was slated to star in the film before his death. (Source.)

Almost every scene in the movie features improvisation from the actors, especially Bill Murray. Particularly amazing is the scene at Louis's party, which is mostly one single shot and is almost entirely improvised by Rick Moranis. (Source.)

Ivan Reitman is the un-credited voice of Gozer and Slimer. (Source.)

When Sigourney Weaver auditioned for the part of Dana, she growled and barked like a terror dog; Reitman told her to "never do that again." (Source.)