Robin Williams and Billy Crystal used to trade funny voicemails in character to cheer one another up

An example of one of Williams' many messages he left for his friend in different characters, said: "Hello, Bill, it's Lord Sisley.

"I'm in Africa. And dear God, man, you should be here. There's creatures who would adore you. I send you all my love. But not like that day in boys school. Something different. Something wonderful. A hug. But if you wish, Bubbly, call."

No matter what was going on in Crystal's life, as soon as he saw he had a message from the 'Jumanji' star, he knew it would make his day.

In the documentary, he recalls: "The phone would ring and I'd look at it and see the 415 area code.

"I knew it was him. I knew it was going to be something really good."

The first time Crystal surprised Williams with a voicemail of his own was when he was in hospital after having heart surgery in 2009.

He made up the alter-ego Vinny the Valve Guy, who he pretended gave the 'Mrs. Doubtfire' star his spare valve.

From that point on the pals relied on each other more and more.

Crystal recalls: "A day and a half [after his procedure] and he was in pain and had just gone through this massive surgery, he called me, 'Oh god, ohhh, hilarious! You know, can I talk to Vinny?'

"As the friendship really grew and grew, we kind of needed each other more."

Filmmaker Zenovich was delighted that she was able to track down the messages to use in the film, as it showed how much Williams and Crystal "loved" one another.

She told The Huffington Post: "So I chased down Billy to get the messages and it took awhile, but one day they appeared on my phone.

"It's like, 'Are you kidding me?!' They were just so wonderful and showed the fun they had.

"I think that they really loved each other and could keep up with each other.

"One line, in particular, I love in the movie is when Billy says, 'Everybody wanted something from him. I just liked him.' You really got a sense of that."