Don't cross the streams
Rick Moranis wasn't supposed to be in Ghostbusters:
John Candy was the first choice for the part of Sigourney Weaver's dweeby neighbor Louis. Candy was interested, but he wanted his character to speak with a German accent and own several large dogs.
Neither were Ernie Hudson or Bill Murray. Dan Aykroyd wrote Peter Venkman for John Belushi, then rewrote it for Murray after Belushi died. I can't even imagine how that would have worked.
(Actually, I don't know if I can easily substitute any other actor for Bill Murray in any of his roles. That might be an imaginative blind spot for me.)
Likewise Paul Reubens (pre-Pee Wee Herman) was originally slated to play the demonic Gozer, as a straight-laced architect in a business suit.
As for Ernie Hudson's Winston:
Eddie Murphy was offered the part of Winston Zeddemore, which was intended to be a much larger character at the time. The plan was for Zeddemore to have been hired as a Ghostbuster much earlier in the movie, and in the scene at the hotel, he would have been the one covered in green slime by the ghost Slimer, instead of Bill Murray.
Murphy's reaction to getting slimed would have been priceless, but going for the lead in Beverly Hills Cop rather than teaming up with Aykroyd again was a great call. It probably all worked out for the best.
In fact, between Ghostbusters, Beverly Hills Cop, and Sixteen Candles, you could make a case that 1984 was the biggest/most important year in modern cinematic comedy. Even Police Academy, Gremlins, Splash, and Romancing the Stone were huge that year, even though I don't like those movies so much.
