One director wanted to turn the story into an animated film.
The movie is based on the Broadway musical of the same name.
Many of the leading cast members were much older than the high-school characters they were playing.
Stockard Channing (Rizzo) was 33, Jamie Donnelly (Jan), was 30, and Olivia Newton-John (Sandy) turned 29 during filming. Newton-John died at 73 on August 8.
John Travolta (Danny Zuko), Jeff Conaway (Kenickie), and Didi Conn (Frenchy) were some of the younger castmates at 23, 27, and 25, respectively.
"Grease" could've been an animated film.
The film's cocreator, Jim Jacobs, told Yahoo News in 2018 that Hollywood director Ralph Bakshi pitched "Grease" as an X-rated cartoon. He was well known for his animated hits like "Fritz the Cat," but Jacobs wasn't sold on his vision.
Instead, producer Allan Carr won the rights and made it a live-action film — but it still featured an animated opening-credits scene.
Henry Winkler was offered the role of Danny Zuko.
Henry Winkler's "Happy Days" character, Fonzie, basically was Danny Zuko, so he was a natural first thought for the film.
During an appearance on "The Rachael Ray Show" in 2019, the actor called it "one of the great decisions" of his life because Travolta "deserved" the role and ended up being so right for it.
And Carrie Fisher was almost in the running for Sandy.
According to Vanity Fair, "Grease" director Randal Kleiser was friends with "Star Wars" creator George Lucas.
When Kleiser was casting Sandy, he watched some footage of Carrie Fisher as Princess Leia to see if she'd be right for the role. But he ultimately couldn't judge her singing or dancing talent and decided to pursue other options.
Danny Zuko wasn't Travolta's first "Grease" role.
Travolta was familiar with the movie's source material before he was ever considered for the role of Danny Zuko.
Newton-John said wasn't sure she could handle the role at first.
Prior to being cast in "Grease," Newton-John was already a popular singer with multiple hits. But she wasn't sure of her abilities as an actress.
During a 1981 appearance on the "The Merv Griffin Show," she said that she wanted to see herself in a screen test with Travolta before accepting the role. That way if she didn't think she was any good, she could drop out.
Fortunately, Newton-John was comfortable enough to proceed as Sandy.
Elvis Presley could've played Frenchy's guardian angel.
According to the same 2016 Vanity Fair article, Elvis Presley was reportedly up for the role of the Teen Angel, which eventually went to "Beach Party" star Frankie Avalon.
Elvis still got a shout-out in the film, but it ended up being a little eerie.
Even though the rock-and-roll legend didn't appear in the film, he was referenced in a lyric from the song "Look at Me, I'm Sandra Dee."
During the film's sleepover scene, Rizzo sings the song and does a little pelvis shake next to a picture of the musician during the line, "Elvis, Elvis, let me be. Keep that pelvis far from me."
Presley died on August 16, 1977, the same day the sleepover scene was shot.
"It was very eerie," Kleiser told the New York Post in 2010. "It was all over the news, so everyone knew. We did this number, and everybody kind of looked at each other like, 'Yeah, this is creepy.'"
There was almost another iconic musical cameo in the film.
Travolta ended up singing the number instead, which differs slightly from the source material.
Even if The Beach Boys didn't sing "Greased Lightnin'," in the stage musical, it was originally meant to be Kenickie's song.
However, Travolta wanted the song for himself and his character, Danny.
"I have to be completely honest with you," Travolta told Vanity Fair in 2016. "I wanted the number. And because I had clout, I could get the number."
Travolta got his sister a small role in the film.
That same clout also came in handy when Travolta asked if his sister, Ellen, could have a small part in the film.
Ellen told The Spokesman-Review in 2018 that she was visiting the set one day when her brother asked if she wanted to be in the movie.
" … So the next day they wrote me in a part," she told the publication. "That's how loose it was, and never knowing the success it would become. I get residuals every 13 weeks for 40 years. And I only worked two days, maybe one day, with Joan Blondell and that group, and I had a lot of fun."
Ellen played one of the waitresses watching the high-school dance contest on TV. Her big line was, "Oh, there's Danny and Sandy."
"Hopelessly Devoted to You" wasn't in the original Broadway production — and neither were a few other songs.
Like many movies that are adapted from Broadway musicals, "Grease" added original songs to its soundtrack — perhaps in the hopes of an award nomination.
Sandy's big solo, "Hopelessly Devoted to You" wasn't in the original Broadway musical. John Farrar, Newton-John's music producer, wrote it specifically for the movie, and it did, in fact, earn the production an Oscar nomination for best original song.
The other original songs from the film are "Grease" and "You're the One That I Want."
Conaway gave Channing a real hickey during filming.
Per Entertainment Weekly, one of the film's DVD extras revealed that actor Conaway actually gave Channing, his on-screen love interest, hickeys during filming.
The production team rented a real carnival for the iconic finale.
Carr didn't skimp on the set details for the film's big finale.
According to Yahoo, the producer rented a fully operational carnival with real rides and everything.
"It was just a big party," Michael Tucci, who played Sonny, told the publication in 2018. "It was like we weren't even shooting a movie; they were just shooting this wonderful experience. And we went on all the rides. I bought Eve Arden [the Golden Age Hollywood star who played Principal McGee] a lot of cotton candy."
Sandy and Danny almost didn't sing "You're the One That I Want"
The big ending number was supposed to be "All Choked Up" — like it was in the original Broadway version.
At the last minute, however, it was switched to the now-iconic "You're the One That I Want."
Kelly Ward, who played Putzie, told Yahoo in the same 2018 interview, "It was lightning-fast work and nobody was really quite sure how it would play, because it's such a different kind of music than the rest of the score. And nonetheless everybody threw themselves into it."
Newton-John had to be sewn into her tight black pants.
Newton-John told The Guardian in 2019 that the costume crew actually had to sew her into them for filming.
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