Grease was a huge hit when it came out in 1978 and has since become an American classic. But when the movie was being filmed in 1977, no one knew they were making what would become the highest-grossing American movie musical of the 20th century, including leads John Travolta and Olivia Newton-John, who weren't the initial choices to play high school sweethearts Danny and Sandy. "Making it was fun but you never know with movies if audiences are going to go with it or not, even if you love it," Newton-John told Forbes. "It is incredible that it is still going but it's not even just that, it's showing no signs of stopping. You say 'Sandy and Danny' and people instantly know what you're talking about."

However, the road to creating the now-classic movie was not an easy one — the right actors had to be found for each part, filming had to be accomplished in difficult conditions, songs were added to and removed from the film and personal dramas were confronted.

Travolta told producers Newton-John was 'every guy's dream'

Grease could've had very different cast members. According to Vanity FairHenry Winkler of Happy Days fame was offered the lead role of Danny Zuko and Travolta only stepped in after Winkler turned down the opportunity. Producer Allan Carr selected a porn star to portray Coach Calhoun until the studio objected (Sid Caesar ended up in the role). Carrie Fisher and Marie Osmond were among those considered for the lead role of Sandy before Travolta suggested Newton-John, then a country singer, for the part. In 2018, Travolta told People magazine what he'd said at the time, "There is only one person that should play this role and it's Olivia Newton-John…She's every guy's dream."

However, Newton-John was reluctant to join the cast. A previous film of hers had flopped and she "was 29 and worried that I didn't look young enough" to portray high school student Sandy. It was only after a successful screen test and interacting with Travolta that she agreed to take the role, telling Entertainment Weekly, "John and I had great chemistry from the first time we met." Sandy became a girl from Australia called Sandy Olsson (she'd been Sandy Dumbrowski in the stage show) to allow the star to use her native accent. 

Lucie Arnaz, Lucille Ball and Desi Arnaz's daughter, was considered for the role of Pink Lady leader Rizzo. However, she has stated she couldn't get a signed contract in time, telling the Austin Chronicle, "I gave up Grease to go do Bye, Bye Birdie at the Melody Top Theater in Milwaukee." Stockard Channing was then up for the role, but there were worries that the 33-year-old actress wouldn't be able to pull off high schooler Rizzo. According to Vanity Fair, the movie's director, Randal Kleiser, decided he'd check all cast members, including Channing, to see if he could spot wrinkles around their eyes. Each of the movie's potential actors had to pass his so-called "crow's-feet test" to officially get the role.

READ MORE: 10 Facts You’ve Probably Never Heard About Grease

Grease cast

(L-R) Jeff Conaway, Olivia Newton-John, John Travolta and Stockard Channing in "Grease"

Filming in hot California posed problems for the cast

Though the stage play was set in Chicago, the movie version of Grease was filmed in California during the summer of 1977. That provided a sunnier backdrop, but it also meant high temperatures. During the filming of the high school dance, the gym became sweltering for the cast. But they couldn't keep windows open for too long because a pork-processing plant was emitting noxious odors that would permeate the setting. Director Kleiser later told Los Angeles Magazine, "People would lie down on the gymnasium floor just to deal with the exhaustion."

A similar problem occurred when the finale, "We Go Together," was filmed on a high school football field. Kelly Ward, who played T-Bird member Putzie, told Yahoo Entertainment in 2018, "There were people who were passing out from dehydration and heat exhaustion and so forth during that sequence. It was an oppressively hot set because there was no place to hide. There was no shade, really. It was high summer and we were outdoors and it was hot."

One day, the actors playing the T-Birds — Barry Pearl (Doody), Michael Tucci (Sonny), Jeff Conaway (Kenickie) and Ward— decided to take advantage of the Southern California location and go sailing during a daytime break in their filming schedule. They needed to be back that evening to shoot a scene and thought they had plenty of time — until the wind stopped and their sailboat was stranded. The group only made it to the night shoot on time when the boat's captain performed a wind-summoning dance that actually worked. Ward told Yahoo Entertainment that the experience was "a major bonding moment for the T-Birds!"

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Olivia Newton-John and female cast members singing "Summer Nights"

The musical numbers were constantly changing

Though Channing got to play Rizzo, she almost didn't get the chance to deliver the powerful song "There Are Worse Things I Could Do," in which Rizzo takes ownership of her sexuality and personal choices. Channing told Vanity Fair that producer Carr considered the song "a downer." She only convinced him it was necessary to include it by arguing, "That’s how you know what’s inside this little person. Otherwise, she’s just all that surface stuff."

Also, the romantic lament "Hopelessly Devoted to You," sung by Newton-John, wasn't included until after filming had finished. And the duet between Newton-John and Travolta at the ending carnival scene, "You’re the One That I Want," was another last-minute addition to the movie. According to Yahoo Entertainment, the choreography was done on the spot. The cast members playing the T-Birds and Pink Ladies ended up lip-synching their lines. Pearl, who played Doody of the T-Birds, later explained to Yahoo Entertainment, "It was a song we had never heard before."

Many of the stage show's songs were cut from the movie. "Greased Lightnin'" was a stage number that did make it into the film, but not without a few changes. Onstage, the character of Kenickie is the one singing about his car, but in the movie, Travolta's Zuko handles the tune. "I have to be completely honest with you," Travolta told Vanity Fair. "I wanted the number. And because I had clout, I could get the number." However, it could've been worse — another plan the production had considered was bringing in the Beach Boys to sing the song.

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John Travolta and the T-Birds singing "Greased Lightnin'"

The 15-week shoot was a 'labor of love'

There was plenty of emotional upheaval going on during the making of Grease. Conaway resented Travolta for taking the song "Greased Lightnin'" away from him. Travolta was still coping with the death of his girlfriend, actress Diana Hyland, in March 1977, and often called Grease's director seeking support. Conaway ended up falling for Newton-John's older sister when they met at a party. For Newton-John, pulling off her character's last-minute transformation from good girl to T-Bird temptress "was an opening up for me. I felt from it that I wanted to try different things. I was open to everything new," she told Rolling Stone.

Amidst these moments of emotion, the actors bonded. Pearl told Yahoo Entertainment, "We all got along. It was 15 weeks of a labor of love. You see it when you watch it." Per Didi Conn, who had the role of Pink Lady Frenchy, "What was so much fun was that most of us were older than the characters we were playing so we just stayed in character all day long," she explained to the Daily Mail. At the end of filming, the cast had enjoyed themselves so much they were ready to make a proposed sequel, Summer School.

However, producers didn't know how big a hit they had on their hands and the proposed sequel wasn't filmed (a few years later Grease 2, starring Michelle Pfeiffer, would get the green light, but it didn't recapture the success or the magic of the original movie). Before heading their separate ways, cast members were able to enjoy themselves one last time at the wrap party. According to Conn, Channing brought some "special brownies" to the event, which she shared with everyone. Conn said in a Daily Mail interview that her recollection of the party is therefore vague: "I really don't remember much, just dancing and lying on the floor laughing with everybody."