Director Ivan Reitman made the big reveal during a cast reunion organised by Josh Gad last week (ends June 12), which aired on YouTube on Monday.
Rick Moranis, who was not part of the get together, eventually nabbed the role of Louis Tully, after Candy passed.
"He didn't really get it," said Reitman. "He kept wanting to play him with a German accent, with a couple of German Shepherd dogs."
"I said, 'I think that would be confusing, given the (demon) dogs that are already on the roof (in the film)'."
Eventually, Reitman sent the script to Moranis, who leapt at the chance to play Tully: "Rick read it and called me immediately and said, 'John just made a terrible mistake, I'm so happy'," the filmmaker added.
Joining Reitman for the "Reunited Apart" cast get together were Bill Murray, Dan Aykroyd, Ernie Hudson, Annie Potts, Weaver, and the director's son Jason, who is currently making the latest instalment in the spooky franchise.