The first sign that this was no ordinary movie screening came the moment the theme music began, and the audience erupted into applause. But that only was the start.

The same thing happened each time a main character first appeared, after famous quotes were uttered and during a scene set seven decades ago featuring Radio City Music Hall — the very place in which the audience sat.

So it went Saturday for the Tribeca Film Festival’s closing night event, a screening of “The Godfather” and “The Godfather Part II,” followed by a panel discussion featuring director Francis Ford Coppola and the films’ biggest surviving stars: Al Pacino, Robert DeNiro, Robert Duvall, James Caan, Diane Keaton and Talia Shire.

Caan credited Coppola with assembling an elite group of actors, as well as the behind-the-scenes crew that helped turn it all into an icon of American pop culture. “Francis somehow knew who had it or didn’t have it,” Caan said.

(Coppola and Caan briefly recalled their time together as Hofstra University students as well.)

The director and surviving cast, most of whom were early in their careers at the time, spoke of the tenuousness of the project, with most of them, including Coppola, in fear of being fired early in the production.

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Coppola figured he was not long for the job, so why not take the risk of casting his sister, Shire, in a key role?

“It was tough for me, but it was tougher for my brother,” Shire said. “During that first ‘Godfather,’ for a few weeks your job as the director was up for grabs and the last thing he needed was a sister who couldn’t figure out where the marks were . . . You didn’t need me to be there, but you were very kind.”

Coppola said a key scene in the sequel in which Keaton’s character tells her husband, Pacino’s Michael Corleone, that she has had an abortion was Shire’s idea.

The studio initially did not want Marlon Brando as Vito Corleone, and it considered dozens of other Michaels before accepting Pacino in the role.

Pacino recalled that he and Keaton got “loaded” after filming the first movie’s famous wedding scene and saying, “We’re done. It’s the worst film ever made.”

Instead, it won the Academy Award for Best Picture of 1972, as did the sequel two years later.

DeNiro, a co-founder of the Tribeca festival, helped organize the reunion of the stars, some of whom had not seen each other or the films in many years.

Keaton said she had not watched in perhaps 30 years, then did so on her computer.

“I was blown out of the ballpark,” she said. “Every scene was astonishing to me and I was totally surprised because I didn’t expect it . . . I was so emotional about it.”