Olivia Newton-John "didn't jump at the offer" to do 'Grease'

Joel said: "It was an incredible movie experience for everybody.

[Casting Olivia] as Sandy was very simple. 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. What if she had said now?"

The casting director went on to explain that Oliva - who at the time of production was known as a pop singer in her native Australia - eventually bargained that she would only take the role if she was allowed to see a screen test of herself and John.

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

'Grease' - which was based on the 1971 stage musical of the same name and tells the story of two- star-crossed lovers at a 1950s high school

- was a box office smash and held the record for the highest-grossing movie musical of all time until it was surpassed by 'Les Miserables' in 2012, with both of its leads going on to enjoy worldwide careers in music and movies.

Following her death, 'Saturday Night Fever' star John paid tribute to the 'Xanadu' actress and remembered her as having had an "incredible"

impact on the world.

He wrote: "My dearest Olivia, you made all of our lives so much better.

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 first moment I saw you and forever!

Your Danny, your John!."