Ackroyd knew Bill Murray from SNL and he'd known Murray's co-star, Harold Ramis, from the hit movie: "Stripes." Not only would Ramis make for a good actor for the movie he could also help them re-write the script. I believe Ramis will go down as one of the best comedic writers of all time considering how many hit projects he's written and directed like: "Caddyshack," "National Lampoon's Vacation," "Groundhog Day," "Stripes," etc. Murray didn't sign a contract, as he was always very aloof, and he hadn't called anyone for a few months leading to filming, so they were hoping he was going to take part in the movie but didn't really know until he showed up for the first day on set.
They only had around ten months to make the movie because Vincent wanted it to be a summer blockbuster. One of their major challenges was that not only did they have to do the filming, they needed all kinds of special effects and there was only one company that did this kind of work (ILM), and they were already tied up on a "Raiders of the Lost Ark," project. The timing was outstanding though as one of the most creative people at their company wanted to start his own business, so he took a bunch of people with him and their first project was "Ghostbusters."
The name "Ghostbusters," was already trademarked in a low budget film at Universal Studios so they weren't willing to give the name up and Ackroyd thought they'd have to call the movie: "Ghostbreakers," which no one wanted. They tried again and again to buy the rights from Universal and they wouldn't budge.
Then during production, Coca Cola purchased Columbia and even though they didn't have a clue about picture making, they told Vincent he needed to scrap the project because they didn't think it would make money. Vincent not liking the new ownership, left and became the CEO of Universal so they picked up the movie.
In the beginning of the movie, the New York skyline and firehouse turned GB headquarters, was done in NYC but most of the movie was filmed in Hollywood at Universal's 400 acre studio area. The film crew didn't have a clue if the special effects were coming together but they were, and everything came together about six weeks before the movie released.
By the way, Ackroyd was shocked that Sigourney Weaver wanted in on the project off of her fresh success with the movie: "Alien." It was her idea for her character to be possessed and turn into one of the evil dogs at the end of the movie. The gatekeeper, actor Rick Moranis (hilarious nerd in the movie), was originally to be played by Pee Wee Herman but the director made a last minute change.
To show how the stars were aligned for this movie, the reason they could finally call it: "Ghostbusters," is that Universal Studios owned the name and since Vincent left Columbia to run Universal, they were able to get their hands on it.
Needless to say, the movie rocketed to fame. The song: "Who You Going Call," went number one. The music video on MTV was being asked to be played again and again.
Last but not least, some executives who were supposed to bring Columbia Pictures into Coca Cola's portfolio, were fired because they cost the company hundreds of millions of dollars by canning the Ghostbuster's project.