Olivia Newton-John 'Didn't Jump at the Offer' to Do 'Grease' — But That's Who John Travolta Wanted

While Olivia Newton-John's performance as Sandy Olsson in 1978's Grease will always be iconic, she was hesitant to accept the role at first, the film's casting director tells PEOPLE.

On Monday, it was announced that the actress died at the age of 73.

"She was just a wonderful, wonderful person," Grease casting director Joel Thurm tells PEOPLE, adding that those who worked on the musical film "were a very close community."

"It was an incredible movie experience for everybody," he said.

Recalling how Newton-John was cast in the role of Sandy opposite John Travolta's Danny, Thurm says that selecting her was "very simple."

For more on Olivia Newton-John, listen below to our daily podcast PEOPLE Every Day.

"John was already pre-set, and John asked me, 'What do you think of Olivia?' I said, 'She's wonderful.' He said, 'What do you think of her for Sandy? That's a great idea.' And once John said that, and that's who John wanted, I stopped anything else."

However, he muses, "What if she had said no?"

Adding that he's known Travolta, now 68, since the Pulp Fiction actor was 17, Thurm says he had "nobody" as a "backup" plan if Newton-John actually declined.

"If she said no, I'd be playing the part in a poodle skirt," he jokes. "So everybody wanted Olivia here, but Olivia didn't jump at the offer. That's the important thing to know."

He explains that the "Magic" singer was "very embarrassed," noting that at the time she was a famous singer in Australia and the U.K. and had just broken into the movie industry.

"She said, 'Okay, I want to see a screen test with John and myself and then I'll let you know if I want to do it,' " he recalls.

Thurm confesses this was a first for him, telling PEOPLE, "I think I've never heard of a case where an actor being offered a role said, 'I want to see me before I say yes.' But that's how smart she was."

RELATED VIDEO: Olivia Newton-John Dead at 73: The Star and 'Grease' Icon Dies of Breast Cancer

He adds that in the first screen test, the jokes didn't quite land with the crew. "We did three takes and no one laughed at the drive-in scene and we're all, 'What is this? What is going on?' "

He switched the screenplay with a copy of the original musical script — the stage musical Grease debuted in Chicago in 1971 before opening on Broadway the following year — recognizing that the "dialogue" for the movie was "different" from the stage production.

After giving the musical's script to director Randal Kleiser, with different lines for Travolta and Newton-John, "five minutes later, the next take, the crew is laughing," Thurm says.

"I honestly can't tell you the difference because, come on, it was 40 years ago," Thurm says, noting that Newton-John and Travolta were also "relieved" to see the crew laughing.

Newton-John agreed to the role after seeing the test on a VHS tape, and Travolta was "thrilled," Thurm tells PEOPLE.

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

Once she accepted the role, Newton-John was hesitant about being older than Travolta, however, their "chemistry was there from the get-go," Thurm says.

"She felt very, very comfortable with him and that was it ... You look at the screen and you see it. He had great respect for her as an artist. And she had the same for him. There was never anything untoward or romantic or stuff like that, but there was a great deal of mutual respect and friendship that lasted forever. They definitely had a spark."

Following the news of Newton-John's death, Travolta and other cast members from Grease paid tribute to the late actress.

Travolta remembered the actress by sharing a heartfelt tribute on Instagram alongside a throwback photo of her.

"My dearest Olivia, you made all of our lives so much better," he wrote in the caption. "Your impact was incredible. I love you so much. We will see you down the road and we will all be together again."

"Yours from the moment I saw you and forever! Your Danny, your John!" Travolta concluded.