Kristen Wiig as the button-down-scientific-type one who really believes but can't say it out loud because she'll get fired from her job as a professor/government scientist/reporter. (I predict a scene where her character gets drunk and finally lets it all loose. This will be accompanied by Kristen drinking a bottle of wine and actually saying "glug glug" as she drinks it.)
Melissa McCarthy as the rambunctious "in-your-face" one. She will fall down a lot and probably punch whoever the asshole government jerk is.
Kate McKinnon as the true-believer psycho one who will shriek "I knew it!!!!" when they encounter their first ghost.
And Leslie Jones as the Tracy Morgan one.