"It was one of the most beautiful friendships to watch grow," Chu, 45, tells PEOPLE of Grande, 32, and Erivo, 38, as he talked all things Wicked at a private dinner hosted by Gold House and OpenTable at the Brooklyn, New York restaurant Win Son on Tuesday, Oct. 28.
"These two are the two biggest voices in the world, maybe in our lifetime. They could have hated each other," Chu adds of Wicked's leading ladies. "There could have been insane divaship, but they always put Wicked bigger than themselves, and I think that's a great sign of a great artist that they could immerse themselves in Elphaba and Galinda and they could find themselves within these characters."
"And I think it only makes Elphaba and Galinda bigger than life, and we get to know them more intimately than ever," Chu added. "I think that's why it feels culturally relevant because of what they're bringing to these characters. It made my job a lot easier."
Erivo and Grande, of course, stepped into roles originated by Idina Menzel and Kristin Chenoweth on Broadway for the big screen adaptation. Last year's Wicked: Part One proved a critical and box office success, but Chu said both he and the movie's stars "were all scared" while shooting the film. "We all knew we just had to throw down and shoot the moon and trust each other," he shared.
"So I loved watching all of that happen. I got the front row seats. I loved them so much," he adds. "They were family. We could have director and actor talks and we could have brother/sister talks, and so that meant the world to me."
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Both Wicked and its high-flying sequel For Good also features Michelle Yeoh, Marissa Bode, Bowen Yang, Ethan Slater, Jonathan Bailey and Jeff Goldblum in its cast. Chu tells PEOPLE he simply wants fans "to sit back, relax and enjoy the ride" when the movie hits theaters Nov. 21.
"This movie allows us to go deeper with these characters than maybe they've ever seen. It's more than just act two," Chu says. "It's all the things that I've always wondered about these characters. And if you thought these girls were good in movie one, wait until you see what they're able to do in movie two. It is absolutely stunning."