“Directing is 90% casting.” This adage, of uncertain origin, has been embraced by some of Hollywood’s most renowned directors, including Robert Altman, Martin Scorsese, John Ford, Elia Kazan, and, notably, Jason Reitman. The filmmaker behind “Saturday Night,” a film chronicling the creation of the inaugural episode of “SNL,” has stated that the success of the movie hinged on getting the casting right.

“He’s gonna find a John Belushi, and he’s gonna find a Gilda Radner, and he’s gonna find a Chevy Chase,” Reitman recently shared with The Hollywood Reporter about the task he assigned to his long-time casting director, John Papsidera. “And, if he doesn’t, I guess we’re not going to make the movie.” With “Saturday Night” now officially in theaters as of Oct. 11, take a look at how the film’s cast measures up to the real-life performers they depict.

“We only talked about a handful, maybe 10 or a dozen, Lornes,” Papsidera told THR about the search to cast the role of Lorne Michaels, the creator of “SNL.” “Once you cross out all the ones that are not Jewish and Canadian, you really come down to one guy,” Reitman added, referring to Gabriel LaBelle, the actor who delivered a breakout performance in Steven Spielberg’s semi-autobiographical film, “The Fabelmans.”

The only obstacle LaBelle faced in casting was his age. Michaels was 30 when “SNL” premiered, while LaBelle is 22. Fortunately, the rising star exudes confidence beyond his years and convinced Reitman he could pull off the role of a 30-year-old.

“I met Gabe standing next to Steven Spielberg, and Gabe held his own standing next to the greatest director alive,” Reitman recalled. “That was one of the first moments where I went, ‘Wow, this kid has a presence.’”

“Matt Wood is a guy who, probably since a teenager, has been going, ‘God, I really hope they make a John Belushi movie, because I’d be perfect,’” Reitman said. And he was right.

“There had to be a couple hundred Chevys that we looked at,” Papsidera recalled. After auditioning actors using skits from the first episode of “SNL,” Reitman realized, “We’re not looking at what the end product is. What we want is the essence of the beginning, so we pivoted.”

Enter Cory Michael Smith from films like “May December” and “Gotham.” “Cory did the scene great and Cory had the voice down, but he also did an amazing fall right at the beginning of his audition,” Reitman recalled. “He did this thing where he stumbled into a trash can.”

“I did no research,” said Kim Matula of her preparation for playing Jane Curtin, before hedging, “I did a little bit of research, but I have been pretty familiar with Jane Curtin already.”

“‘SNL’ was such a big part of my household growing up,” she added. “So I knew immediately I was like, I know how Jane delivers this.”

“Who thought Gilda would be a British actress that starred in Dickinson?” Papsidera quipped to THR. Not Ella Hunt. She thought there was no point auditioning for the role. But her agent dared her, so she did. And she booked it.

Casting Dan Aykroyd proved a challenge for Reitman, given that he had grown up knowing the actor. Reitman’s father, Ivan Reitman, directed Aykroyd in the original “Ghostbusters” movie.

Garret Morris, the only Black performer among the original “SNL” bunch, who also happened to be older than his cohorts, was the first character Reitman cast.

Braun was first cast as “Muppets” creator Jim Henson and ended up pulling double duty on “Saturday Night” after Reitman asked him also to play the role of legendary comedian Andy Kaufman.