Liam Neeson and Steven Spielberg at the ‘Schindler’s List’ screening at the Tribeca Film Festival. Photo: Alan Zeitlin.

Shooting “Schindler’s List” took an emotional toll on director Steven Spielberg, but he got by with a little help from a friend. Spielberg told a sold-out crowd at the Beacon Theater on the Upper West Side of Manhattan that during the shoot, comedian Robin Williams would call him once a week from San Francisco.

“He would do 15 minutes of stand-up on the phone, and I would laugh hysterically cause I had to release so much,” Spielberg said. “But with the way Robin is on the telephone, he’d always hang up on you on the loudest best laugh you’d give him,” without even saying goodbye.

Spielberg, who was joined by other actors from the film in conversation with New York Times critic Janet Maslin — as part of The Tribeca Film Festival — responded to a recent survey showing that millennials didn’t know much about the Holocaust. Spielberg said that while he thinks a good amount of attention was paid to the Holocaust at the time of his film — and that he started the Shoah Foundation to disseminate the stories of survivors — it is up to schools to do more.

“It’s still a voluntary educational option,” Spielberg said. “It’s not a pre-requisite to graduate high school, as it should be. It should be part of the social sciences, social studies curriculum.”

As Spielberg and the cast gathered to mark the 25th anniversary of the film’s release, he said it was true that Martin Scorsese was at one point a possible director for the film — but said it was false that Mel Gibson was considered for the leading role. The role of Oskar Schindler, the man who saved 1,200 Jews from death, of course went to Liam Neeson.

Neeson sat to the right of Spielberg, and spoke about how when he was asked to put on weight, he tried protein shakes — which caused him to vomit. But Guinness, he said, worked much better. Neeson said that he finished a play in New York on a Sunday, and on a Wednesday, he was on set at 5:30 a.m. at the gates of Auschwitz. It was freezing, and he was nervous with guard dogs that weren’t acting.

Neeson said that it hit him “big time,” when producer Branko Lustig pointed to the hut that he’d been in when he was a prisoner at Auschwitz.

 “My knees were literally shaking,” Neeson said, adding that he kept getting his line wrong, in which he explained that little children were necessary for his factory because they had small fingers that were able to clean metal shell casings.

Neeson said that he’s done more than 55 movies, but never saw a director like Spielberg, who didn’t use storyboards as other directors did.

“I’ve never experienced a director so nervous and so almost terrified as Steven Spielberg was in the early days of shooting ‘Schindler’s List,’” Neeson said. “And I’ve likened him to an abstract painter with a blank canvas with this energy and this story he has to tell … it has to come out, and he has a potpourri of colors and he has a brush and he doesn’t know which color to start with. But once he has that color on the brush and has put it … to the canvas, then he was just going for it.”

There were also a few surprising tidbits.

Seeing actor Ralph Fiennes in his Nazi uniform on set, a Polish woman in an overlooking structure called out that she would like the Nazis to come back and protect them, Spielberg said, adding that swastikas were waiting for them as they drove to work.

Ben Kingsley, who played Itzhak Stern, confronted an antisemitic businessman who mimed a noose to a cast member, after asking for and getting confirmation that he was Jewish.

“And I stood up,” Kingsley said.

“You did more than that,” Spielberg quipped.

Spielberg, who said that the screening was the first time he’d seen the film with an audience since it came out 25 years ago, told the crowd how he couldn’t talk about past films with the German actors at first.

“The uniforms stopped me,” he said.

But then someone arranged a Passover Seder, where German and Israeli cast members sat next to each other and traded off reading from the same Hagaddah.

“I started crying,” Spielberg said. “That broke it for me. And the next day, I talked about ‘ET,’ ‘Raiders of the Lost Ark,’ ‘Close Encounters,’ whatever they wanted to talk about.”