The HBO documentary Robin Williams: Come Inside My Mind will debut at the Sundance Film Festival and then on HBO in the fall.

David Letterman Shares His First Impressions of Robin Williams in Clip from New Documentary

A new documentary is taking an intimate, heartfelt look at the life and career of Robin Williams.

Told largely by the comedian himself, HBO’s Robin Williams: Come Inside My Mind peels back the curtain on the unique entertainer’s incredible time in show business. The film also features a wealth of never-before-seen footage and in-depth interviews with many of his famous friends, including Billy Crystal and David Letterman.

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In a clip exclusive to PEOPLE, Letterman recalls the first time he saw Williams onstage. “When we first saw Robin onstage, we were skeptical,” the talk show host says. “In my head, my first sight of him was that he could fly because of the energy. It was like observing an experiment.”

“I wanted to make this film because I grew up loving Robin Williams, and was really curious about how he evolved into the beloved comedian and actor we all knew and adored,” Marina Zenovich, the documentary’s director tells PEOPLE.

Image zoom Robin Williams Dan Steinberg/Invision/AP

“My goal was to show all the facets of Robin’s life, beyond just what audiences saw on the stage,” she added. “I got such a kick out of interviewing Robin’s friends like David Letterman, Billy Crystal and Elayne Boosler, who had such incredible memories of Robin as a person and performer.”

Also interviewed were Monty Python’s Eric Idle, Whoopi Goldberg, Steve Martin, Pam Dawber and Zak Williams, among many others.

Williams passed away at the age of 63 in 2014 after suffering from Lewy Body Disease, a type of dementia that effected his thinking, memory and movement control.