While Bill Murray is much loved, he's also rather crotchety and so his exact involvement in the planned Ghostbuster 3 has always been up in the air. After all, it was difficult enough to get him to come back for part 2 (and he's never been particularly complmentary about that movie).
While Sigourney Weaver recently suggested that he'd be a ghost in GB3, all we definitely knew was that for the film to be made, Murray had to approve the script (as did the other main actors, director Ivan Reitman and Sony), and even if he was a ghost, that didn't mean he'd look like the actor - he could have come back as slimer for all we knew.
However, now Murray has been speaking to the Mail On Sunday and told them, "I said to them, 'I'll do it if you kill me off in the first reel.' So now they are going to have me as a ghost in the film." Unfortunately he didn't go on to reveal much more, but it does sound like he'll be making a cameo in a few scenes at least.
The latest rumours are that the film, which will see the old ghostbusters hand over the reins to a bunch of younger guys, will feature Peter Venkman's son from Ghostbusters 2, Oscar, now all grown up and taking on his dad's mantle. With Murray being a ghost, it sounds as if there'll be a few father-son issues to take care of.
At the moment there's still no guarantee the movie will be made, but Sony seems keen to resurrect the franchise. It won't be the same if the main focus is on a new gang of Ghostbusters, but at least the old guys will still be around, even if one of them is in spectral form.