PARK CITY, Utah – The most star-studded entry at the Sundance Film Festival is a short documentary about an actor whose intense face is known to just about any serious cinema fan but whose name often escapes them.

“I Knew It Was You” features interviews with Al Pacino, Meryl Streep, Robert De Niro, Gene Hackman, Richard Dreyfuss, Francis Ford Coppola and others, all paying tribute to the guy whose long, sad face leaves people saying: “I know him. Isn’t that Fredo from ‘The Godfather’?”

John Cazale made only five films, among them the first two “Godfather” flicks, before dying of cancer in 1978 at age 42. But all five were nominated for best picture at the Academy Awards, including “Dog Day Afternoon,” “The Conversation” and “The Deer Hunter.”

“Five best-picture nominations. A perfect record,” said Richard Shepard, who directed “I Knew It Was You,” a 40-minute documentary that premiered in Sundance’s short-films program and eventually will air on HBO. “It’s insane, isn’t it?”

The acclaimed theater actor became one of Hollywood’s premier character actors with his five films, starting with Coppola’s “The Godfather,” in which he played hapless Fredo, the weak link in the Corleone crime family.

The immortal line from Pacino’s Michael Corleone to older brother Fredo in “The Godfather Part II” – “I know it was you. You broke my heart” – gave the documentary its title.

Cazale broke hearts on screen with portrayals of volatile, vulnerable, vacillating men, including Pacino’s tragic bank-robbing partner in “Dog Day Afternoon.”

“He’s fearless, because he’s not worried about looking good. A lot of actors are so conscientious about being the GUY. They’re afraid to look weak, always trying to have the bravado, have the coolness,” said filmmaker Brett Ratner (“X-Men: The Last Stand,” the “Rush Hour” flicks), one of the documentary’s producers. “He was OK being vulnerable. I don’t know; he just touched me, his humanity. I fell in love with it. In ‘Dog Day Afternoon,’ it broke my heart. … I cry, weep at the end.”

Shepard, director of the hit man tale “The Matador,” said Cazale is his favorite actor, and the documentary got its start out of his frustration at being able to find so little information about him on the Internet and in film books.

“It’s ridiculous, and it just infuriated me,” Shepard said. “I was like, I’ve got to do something about it. Someone should make a movie.”

So Shepard started making a movie, along with producer Stacey Reiss. They landed interviews with Streep, “Dog Day Afternoon” director Sidney Lumet and Cazale’s brother. Then they approached Ratner because Shepard recalled reading that the filmmaker also was a Cazale fan.

Ratner secured financing from HBO. Then the floodgates opened as one top Hollywood name after another jumped at the chance to share recollections and observations of Cazale.

Besides the actor’s many collaborators, the documentary includes interviews with such Cazale admirers as Philip Seymour Hoffman, Steve Buscemi and Sam Rockwell.

“What was amazing was how many people wanted to do it,” Reiss said. “These people get asked all the time to be in documentaries, and it was just an outpouring of support. I think they all were just so happy that other people were going to get to know this person that they cared about and loved so much.”