The filmmaker first worked with Leonardo in 1996 and wanted to reunite with him for another project. However, he knew convincing the 41-year-old to appear in 2013's The Great Gatsby remake wouldn't be easy.

"He doesn't make those decisions lightly, and he put me through the hoops in a really good way," he tells iHeart Radio's Elvis Duran. "It really was about working with Leonardo again and we were always trying to think of something to do and I had to wear him down."

"(His) decision making process (for Romeo + Juliet) was worse," he adds. "It took longer! But honestly, I admire him for that because the thing about Leonardo, I mean he's a friend, our families are friends, is that he looks for joy in life, but when it comes to two things, acting and the environment, he just doesn't take it lightly."

Baz knew he wanted to collaborate with Leonardo on Romeo + Juliet when he first saw the actor because he physically matched what he was looking for in his Romeo, but the director wasn't sure if he could do the role justice.

He recalls, "I saw Leonardo, and then I thought, 'God, that's how he should look but can he act?' Then I found out he was the preeminent young actor."

And Leo surprised Baz when he made a trip to his native Australia for an impromptu audition.

"He came down to Australia for nothing and he brought all these friends and we made a whole video of the show with other actors," he continues. "I won't say who, it will never be seen, (but) I realised that I couldn't make it without him."