Making the 1993 drama Schindler’s List was traumatic for Steven Spielberg—but the director could always count on Robin Williams to make him laugh. At a 25th anniversary screening and panel about the film at the Tribeca Film Festival, the Oscar winner warmly reminisced about the late comedian, whom Spielberg befriended during the making of Hook, released two years before the Holocaust epic. Williams, Spielberg said, was keenly aware of the toll Schindler’s was about to take on his friend—so he made it a habit to regularly call Spielberg and cheer him up during the brutal shoot in Poland.

“Robin knew what I was going through,” Spielberg said. The comedian would call him once a week, always at the same time. “He would do 15 minutes of stand-up on the phone. I would laugh hysterically . . . he’d always hang up on you on the loudest, best laugh you’d give him. Drops the mic, that’s it.”

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Stories about Williams’s phone calls have long been part of the lore of Schindler’s List, which was represented at the Tribeca panel by Spielberg, Sir Ben Kingsley, Liam Neeson, Embeth Davidtz, and Caroline Goodall, and moderated by Janet Maslin of The New York Times.

Back in 2014, Williams himself confirmed rumors of the calls while answering a fan question during a Reddit A.M.A.

“I think I only called him once, maybe twice,” Williams said. “I called him when I was representing People for the Valdheimers Association. A society devoted to helping raise money to help older Germans who had forgotten everything before 1945. I remember him laughing and going ‘thank you.’”

Williams wasn’t the only one helping Spielberg get through the making of Schindler’s, though. During the panel, Spielberg noted that the cast and crew also “watched a lot of Saturday Night Live” to wind down after grueling days on set. At the time, Spielberg also had to make time to approve finishing touches on Jurassic Park, a film he shot earlier in the year. But watching footage of giant dinosaurs didn’t provide the same relief or distraction as a stand-up set from Williams. Spielberg actually came to resent the film—in the moment, anyway, when compared to the dramatic work he was currently doing.

Schindler’s List, a devastating retelling of the Holocaust and the heroic life of Oskar Schindler, was so realistic that it had a strong psychological impact on the filmmaker, as well as its cast. While shooting the scene where the female characters are marched into what turns out to be an actual shower—though they believe it will be a gas chamber—two actresses had breakdowns, and couldn’t resume filming for days afterward, Spielberg recalled. The director also cited the horrifying health-action scene, in which the characters had to strip naked and be judged by Nazi doctors, as “probably the most traumatic day of my entire career.”

“There was trauma everywhere,” Spielberg said. “And we captured the trauma. You can’t fake that.”

There was also trauma behind the scenes, the director said. Spielberg remembered seeing fresh swastikas painted on building walls along the cast and crew’s route to work, and an older Polish woman who happened to live nearby telling Ralph Fiennes, who was dressed in his character’s Nazi uniform, that she missed the days when the actual Nazi soldiers were “protecting” the community. The cast also recalled the night when an anti-Semitic German businessman went up to actor Michael Schneider at a bar and asked him if he was Jewish.

“Michael, in shock, said yes,” Kingsley remembered. The man then “mimed a noose around his neck . . . and pulled it tight. And I stood up.”

“You did more than stand up,” Spielberg said, noting that Kingsley actually “took him right to the ground.”

In stark contrast to that swirling cruelty, Neeson, who played the title role, recalled the times he and Kingsley would wait up for some of their Jewish cast members after particularly arduous days, doling out hugs and free drinks. “Those were lovely evenings,” he said.

After its release, Schindler’s List would go on to win seven Oscars, including best picture and best director. Spielberg later set up the U.S.C. Shoah Foundation, a nonprofit that has recorded thousands of stories of Holocaust survivors. At the screening, the filmmaker actually sat down to watch his film with an audience for the first time in years, and said he came away feeling immense pride at his achievement. Goodall, who marveled at the film’s beauty all these years later, perhaps said it best: “Every scene was a tiny masterpiece.”