Every time one of the original Ghostbusters stars promotes a film, they answer the inevitable Ghostbusters III question that fuels a new round of breathless stories that turn out to be apparitions. Ivan Reitman, back as director thanks to his strong original Ghostbusters deal, is the latest to weigh in as he promotes his latest film No Strings Attached. He is getting more mileage telling reporters that Bill Murray doesn’t hate the script than he is talking up the Natalie Portman-Ashton Kutcher sex romp. Here’s what I hear: Reitman and Sony Pictures have no idea if the mercurial Murray will smile on the script they’ve just sent him. Without Murray, the studio absolutely will not make the film, which gives an extraordinary amount of leverage to a quirky actor who leaves financiers sweating until the moment he shows up on the set.

This is too big a project to play games. “The studio won’t even think about forward on a $150 million film unless Bill has a closed deal and a commitment,” said an insider. “It’s too huge a risk to do any meaningful prep, hoping he shows up.”

It has been 21 years since the last Ghostbusters sequel, which wasn’t as good as the 1984 original and didn’t gross as much. Despite that, Sony Pictures Entertainment has tried over and over to scare up a new ghost comedy that would entice Murray and his compatriots Dan Aykroyd and Harold Ramis for another encore. It has gone long past the point when a studio hurries a sequel to capitalize on the momentum of the last film and nobody was going to hurry unless a good script made it worthwhile, Sony has proven adept at figuring out problem sequels: for years, Men in Black 3 seemed unmake-able because of the battles between director Barry Sonnenfeld and producer Walter Parkes, a prohibitive amount of first dollar gross committed, and the inability to agree on a creative direction. When a good script finally materialized, Sonnenfeld and Parkes made peace and talent traded first dollar gross for cash break deals. The picture is slotted for May 25, 2012 release.

SPE thinks it has the same opportunity on Ghosbusters III, because of the script by The Office writers Lee Eisenberg and Gene Stupnitsky. Murray had been aloof about an Ghostbusters but Aykroyd and Ramis felt a thaw after the cast got together a couple of years ago to work on a video game. Eisenberg and Stupnitsky delivered the goods, and Aykroyd, Ramis and Reitman added their own touches before sending the script to Murray. The script brings back the original cast, who pass the torch to newcomers, which sets up future installments.

When the mercurial Murray does turn up on a film set, it’s worth the wait. He’s generating Best Supporting Actor talk for his performance in Get Low, and he was a definite highlight of Sony’s Zombieland. But nobody envies a studio having to rely on him for such a big piece of business, because Murray moves to the beat of his own drum. Even trying to get an answer on Murray’s thoughts conjures up the signature Ghostbusters line, ‘Who you gonna call?’ He has been without an agent for several years and doesn’t have a publicist. He has an attorney but a call to his office was met with the curt reply that they don’t talk to reporters. Bottom line: a yes from Murray moves Ghostbusters III right into pre-production, with shooting to begin as fast as possible. If Murray says no, the film doesn’t have a ghost of a chance. No pressure, Dr. Peter Venkman.