+Kristen Wiig is playing Erin Garber, a published author on the subject of the paranormal. Erin now works for Columbia University.
+Leslie Jones is playing Abby Bergman, who co-authored the book with Garber. Since publication, the two former colleagues had a falling out and went their separate ways. Abby more actively chases ghost a la Ghost Hunters. This tension between academic investigation (Wiig's character) and mainstream media investigation (Jones' character) is central to the story.
+Kate McKinnon is playing Abby Berman's new partner, Jillian.
+Melissa McCarthy is playing Patty, an NYC subway worker who stumbles across the main, supernatural threat in the film.
+Sony wants Peter Dinklage to play the main villain in the film. Whether this role is as a ghost or human is unclear.
+Sony also wants Bill Murray to play a character named Martin Heiss, a man who is "a professional supernatural debunker who is out to prove that the Ghostbusters are frauds."
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Looking at the above character descriptions it seems like Jones is Murray, Wiig is Ackroyd, McKinnon is Ramis, and McCarthy is Hudson.