'Ghostbusters' at 35: Ivan Reitman on casting Sigourney Weaver over Julia Roberts, the frightening challenge of Marshmallow Man and more

Kevin Polowy

The genesis of Ghostbusters has been well documented in the 35 years since its release.

The project originated in the early '80s as a treatment the paranormal-obsessed Dan Aykroyd wrote for himself and fellow SNLer and Blues Brother John Belushi to costar in. Belushi, though, died of a drug overdose in March 1982.

"It was impossible to make once he passed away," director Ivan Reitman told Yahoo Entertainment in a recent interview commemorating a new 35th anniversary steelbook 4K/Blu-ray release (watch above). Reitman, however, was still intrigued by its prospects, even if the original more "fantastical" version had to be scaled down for budgetary reasons — or literally brought back down to earth. Aykroyd's original version had the guys — ultimately played by himself (Raymond Stantz), Bill Murray (Peter Venkman), Harold Ramis (Egon Spengler) and Ernie Hudson (Winston Zeddemore) — traveling through space and time to bust ghosts.

"I thought there was this wonderful central idea to it, which was there was a group of people who functioned much like firemen whose job was to rid their environment of ghosts."

Also deeply steeped in Ghostbusters lore is the list of actors who almost starred in the film, names like Richard Pryor (Venkman), Eddie Murphy (Winston), John Candy (Louis), Sandra Bernhardt (Janine) and Paul Reubens (Ivo).

"Because there was a five- or six-year period after Belushi passed away [before the film was made], I think [Aykroyd] spoke to a number of people about perhaps doing a duo with him," Reitman explained.

There was one future A-lister, though, whom he did encounter in the casting rounds: "Julia Roberts came in and auditioned," he said of finding the film's lead female role, Dana Barrett. Roberts was a complete unknown at the time, at least four or five years before she broke out in films like Mystic Pizza (1988), Steel Magnolias (1989) and Pretty Woman (1990). She ultimately lost the part to Sigourney Weaver.

"I thought Sigourney was perfect, and I think one of the lovely things we found as we were playing together on the set was the kind of great chemistry that developed between Bill and Sigourney."

Casting the film's lifelike ghouls was also key. And though many of the original creatures Aykroyd dreamed up in that first script (like a Godzilla-esque ghost lizard) didn't make the cut, perhaps his most ambitious creation survived.

"One of the things that did originate in Aykroyd's very [first] treatment, along with dozens of other ghosts that never made it into our film, was the Marshmallow Man.

"And the Marshmallow Man was the thing as a director that I was most frightened about… whether that was really pushing the reality level envelope a little too far. It was the thing that I was most nervous about going into post-production, but it turned out to be this most magical moment."

Ghostbusters/Ghostbusters II 35th Anniversary Steelbook will be available June 11. Get it on Amazon.

Read more on Yahoo Entertainment:

Want daily pop culture news delivered to your inbox? Sign up here for Yahoo Entertainment & Lifestyle's newsletter.