Not to veer too far off topic, but to follow up on this: the version of GB Aykroyd had been writing all those years bears almost zero resemblance to the version we now know. (Reitman, on Aykrod's forty page "first draft": "It took place on a number of different planets or dimensional planes. And it was all action. There was very little character work in it.")
Reitman paired up Akryoyd with Ramis and the instruction to set it in a modern context. He then pitched the basic conceit -- without a script -- to the studio. He told them who was involved, and they asked how much it would cost. The studio said if they could make the picture for 25 million and have it ready for release in summer of 1984, they'd have a deal. At that point, with no script, they had exactly a year to complete the movie for release.
If you can get your hands on the book "Making Ghostbusters", its an incredibly detailed look into the production.