Sigourney Weaver: "I once had a fire in my apartment [after the movie], and the firemen came to put it out. One of them opened up my refrigerator and said, 'Whoa, you better call the Ghostbusters.'"
Julia Roberts auditioned for the role of Dana Barrett in "Ghostbusters" (1984), but it was Weaver who attracted the filmmakers' attention. There was some resistance to casting her because of the generally serious roles she played in "Alien" (1979) and "The Year of Living Dangerously" (1982). Weaver revealed her comedic background, developed at the Yale School of Drama, and began walking on all fours and howling like a dog.
It was Weaver's suggestion that Dana become possessed by the demonic dog, Zuul. Reitman said this solved issues with the last act by giving the characters personal stakes in the events. Weaver said kids would come up and ask her if she really turned into a dog.
Weaver also changed the role from that of a model to a musician, saying she can be kind of strict, but "you know she has a soul because she plays the cello."
Weaver floating is an actual physical effect. The actress was put in a full body cast and attached to a post hidden in the curtains. According to the DVD commentary, the effect came from Ivan Reitman's Broadway experience.
