Among the films premiering at the 2018 Sundance Film Festival is “Robin Williams: Come Inside My Mind,” a new documentary about the late comedian’s life and career.

At a Q&A following a screening of the documentary, David Steinberg, an executive producer of the film who was also Williams’ manager for 35 years, said the comedian was actually “a very, very shy person” in real life.

“When he wasn’t performing or with close friends, he was very into himself,” Steinberg said.

Though Williams’ exuberant performance in movies like “Mrs. Doubtfire” and “Dead Poets Society” may seem to suggest otherwise, this surprising fact was one of the themes of the new documentary.

This trait contributed to Williams’ comedy and stemmed from his childhood, when he had a great deal of time alone with which to entertain himself, according to Steinberg.

“He had to create his own world, and he used to be the soldiers and the commanders and the enemy, and that’s where I think his vocal dexterity came from, from having to really humor himself. He was all things inside his head,” Steinberg said. “When he was older, he was exactly the same way inside his head, except when he had an audience.”

Williams “loved lonely things,” including hobbies such as bike riding, long-distance running, video games and cross-country skiing, and he was also a “voracious reader,” according to Steinberg.

“He was the best-read person I’ve ever known,” Steinberg said. “He was a student of war, he was a student of the army, he knew about physics, he was into science; he was the most informed person that I’ve ever known.”