Olivia Newton-John won countless hearts as Sandy in the blockbuster film "Grease" – but it almost didn’t happen.

"John [Travolta] was set before anybody else was even attached to the movie," casting director Joel Thurm told Fox News Digital about the 1978 musical. "Paramount figured that they could do ‘Grease’ if John was in it. That was a given."

"John really wanted Olivia for the role of Sandy," Thurm shared. "We talked about it and I said, ‘It’s a great idea.’ But my problem was to make Olivia work. Olivia did not want to do it initially. She didn’t leap at this opportunity."

Joel Thurm's book cover

Joel Thurm recently wrote a tell-all titled "Sex Drugs & Pilot Season: Confessions of a Casting Director." (BearManor Media)

Thurm shared his account of bringing the beloved film to life in a new memoir titled "Sex, Drugs & Pilot Season: Confessions of a Casting Director," where he dished on his time in Hollywood.

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According to Thurm, the singer was "embarrassed" and "humiliated" over how she came across in a British science-fiction movie she made years before titled "Toomorrow." The 1970 film told the bizarre tale of aliens that kidnapped the pop group Toomorrow, whose "vibrations" from their instruments are needed for survival.

"She met John and she liked him," Thurm explained. "She also liked the role of Sandy, but she was really cautious. She said, ‘I was embarrassed by the last movie that I did, and I don’t want that to happen again.’ By this time she’s a huge worldwide star and busy on tour. But we all thought she was perfect, that she was Sandy. So, she said, ‘All right, well, in that case, I’d like a screen test.’ That is the first and only time I’ve ever heard of an actor or actress asking for a screen test."

A black and white photo of John Travola and Olivia Newton-John smiling

John Travolta wanted Olivia Newton-John – and Olivia Newton-John alone – to play Sandy in "Grease." However, the singer needed some convincing. (Frank Edwards/Fotos International/Getty Images)

There were other reasons why Newton-John was hesitant about saying yes to Sandy.

"Olivia didn’t consider herself an actress and was wary of acting in general and taking on a leading role in a high-profile film opposite, in her view, a ‘much’ younger man," Thurm wrote. "She was 28 and Travolta, 23. She really should have had no worries about that. Forty years later in her Sandy costume, she looked remarkably the same up until her passing."

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Immediately, Thurm and his team prepared for Newton-John’s screen test in hopes of finally winning her over. The scene they chose for her was at the drive-in where Danny (Travolta) makes a move on her, and she rebuffs his advances. What was supposed to be a comical scene was met with silence.

Olivia Newton John and John Travolta in costume for Grease

Olivia Newton-John insisted on having a screen test to prove she was the right choice for the role of Sandy. (Paramount Pictures/Fotos International/Getty Images)

"The first take, there was no laughing from the crew," Thurm recalled. "And this wasn’t a little screen test. This is a full movie crew on a sound stage. No laughter. Second take, no laughter. Third take, no laughter. At this point, I’m getting really worried. I’m thinking, ‘Oh God, if she thinks it’s her fault that nobody’s laughing, what’s going to happen?’"

"I immediately pulled out from my back pocket a copy of the original ‘Grease’ musical that was running in New York," Thurm continued. "I went to the same scene and read it. I said, ‘Here’s why it’s not working.’ The movie dialogue was all changed. So I gave the play dialogue instead to John and Olivia. They looked it over, and in the next take, the crew was roaring with laughter. The second take, same thing. So we knew we were home. The tapes were sent to Olivia and John. But by the time we were looking at it in a screening room, Paramount executives already saw it and were thrilled. Olivia was perfect for the role, as we all already knew. And thankfully, Olivia liked it too because we had no backup for the role. She was it."

There was one person who wasn’t jumping for joy over the dialogue change – producer Allan Carr.

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Olivia Newton-John wearing neon fashion at Studio 54 in New York City

Olivia Newton-John and John Travolta's song "You’re the One That I Want" was one of the era’s biggest songs. It sold more than 15 million copies. (Ron Galella/Ron Galella Collection via Getty Images)

"Allan asked me if I was responsible for the dialogue switch," said Thurm. "Like a little kid, I said, ‘Yeah! And wasn’t it great?’ He just glared at me and walked away. And from that moment on, I was on his s--- list because I changed his dialogue. But the truth is, he screwed up with that dialogue. It wasn’t right for John and Olivia. It just wasn’t as good or funny as the stage version. That, plus a little improvisation from the actors is what you see in the movie. There’s virtually none of the original movie dialogue in the actual movie."

"As for Olivia, when I think of her, I think of how smart she was," Thurm shared. "She didn't just take the role. She wanted to prove herself. That’s how she maintained her position as a worldwide pop star for so many years."

The "Hopelessly Devoted to You" songstress later recalled how she didn't immediately say yes to the role of a nice girl who toughens up in the final act and gets her man.

A scene of Olivia Newton-John and John Travolta dancing in Grease

Olivia Newton-John is still remembered as everyone’s favorite Sandy in the blockbuster film version of "Grease." (Paramount Pictures/Fotos International/Getty Images)

"I worried that at 29 I was too old to play a high school girl," Newton-John told The Telegraph in 2017. "Everything about making the film was fun, but if I had to pick a favorite moment, it was the transformation from what I call Sandy 1 to Sandy 2. I got to play a different character and wear different clothes, and when I put on that tight black outfit to sing ‘You’re the One That I Want,’ I got a very different reaction from the guys on the set."

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Thurm said the star never lost her sparkle as Sandy.

"In 2019, just before COVID, they showed the movie in Florida and afterward, John and Olivia came out in costume," he said. "The time we spent together was absolutely wonderful. She was incredibly kind to everybody. She made sure that at every event, she would connect with those in the audience who had questions about cancer or might be going through the same thing."

Olivia Newton-John and John Travolta smiling and embracing each other years after Grease came out

Olivia Newton-John and John Travolta attend the "Grease" 40th anniversary screening at Samuel Goldwyn Theater on Aug. 15, 2018, in Beverly Hills, California. The actress passed away in 2022 at age 73. (Alberto E. Rodriguez/Getty Images)

"And I think that was part of why she lived so long with this disease," he shared. "I think it really helped her to talk about it. It helped her when she knew she was helping others… Her spirit was always up. She was just an incredibly special human being. I miss her."

The entertainer passed away in August 2022 at age 73 following a lengthy battle with breast cancer.

The Associated Press contributed to this report.