​​Billy Crystal reflects on friendship with Robin Williams 4 years after his death (Exclusive)

Gone but far from forgotten.

The beloved actor and comedian Robin Williams died almost four years ago, but his legacy lives on in the new documentary Robin Williams: Come Inside My Mind. And, while walking the red carpet at the film’s premiere in Los Angeles on Wednesday, his close friend, Billy Crystal, discussed Williams with ET's Nischelle Turner.

“It's gonna be four years in August and it just-- it feels like a second ago, but I just hope that people will come and see this movie and understand a little bit more about the man and what made him this explosively funny genius,” he said.

Crystal also reflected on the private Williams that fans rarely saw, which the documentary explores. “Well, you know, when they see somebody in a movie, or in a concert or in six minutes on a talk show or whatever, that's what they get. And this film gets you behind that and lets you get to understand how that happened. You know, how that kind of mind was created.”

Crystal added that, early on, Williams was a “lonely kid, a kid on his own a lot, inventing characters, inventing whole worlds and as he grew, it got more complete and easier for him to get to.”

When asked about Williams’ battle with depression that ultimately resulted in his tragic suicide, Crystal honored his friend with a tight-lipped response.

“You know, that's really stuff I don't talk about. Incredibly difficult times,” he said. “You know, friends trusting each other with the darkest of secrets that I'll keep that way.”

The 70-year-old comedian also explained his playfully contentious dynamic with Williams when they did interviews together. “He always looked to me to bring him back to earth and I could take him in a different direction and it took me a while to figure out how to counter-punch and score my own points, you know… it was a privilege to trade blows with him.”

Robin Williams: Come Inside My Mind airs July 16 on HBO at 10 p.m. ET/PT.

See Crystal's full interview in the clip below.

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