In the documentary “I Knew It Was You: Rediscovering John Cazale,” the film’s director, Richard Shepard, showed people on the street a publicity still from “The Godfather” featuring Cazale, Marlon Brando, Al Pacino and James Caan, pointed to Cazale and asked, “Do you know who this guy is?”

Most folks shook their heads and walked away. Others answered, “That guy who played Fredo in ‘The Godfather.’”

Their reactions hurt my feelings. However, identifying Cazale with that iconic role makes sense. He appeared in only five films – “The Godfather,” “The Godfather Part II,” “The Conversation,” “Dog Day Afternoon” and “The Deer Hunter” – and he played Fredo in two of those films.

And Fredo was a great role. The documentary, an HBO production I finally saw thanks to Netflix, gets its title from that classic line in “Godfather II” when Michael kisses his brother and says, “I know it was you, Fredo. You broke my heart. You broke my heart.”

Fredo broke everyone’s heart. That was the beauty of Cazale. Fredo betrayed his brother, yet our – meaning my — sympathies lay with Fredo. As noted in the documentary, Cazale played characters you shouldn’t like, but thanks to a vulnerability Cazale wasn’t afraid to show, you root for these lowlifes.

Sal the bank robber in “Dog Day” was such a character. The guy’s creepy and eager to kill hostages. Yet, we – meaning me – hope he gets away. According to “Dog Day” director Sidney Lumet, Cazale improvised the movie’s famous Wyoming line.

Sonny: “Is there any country you want to go to?”

Sal: Long pause, “Wyoming.”

Sonny: “Sal, Wyoming’s not a country.”

Sad and funny simultaneously, that was another Cazale trait. His “Deer Hunter” co-star and lady love Meryl Streep said there was tragedy in the most comedic of Cazale’s characters and comedy in the most tragic.

Stan, his character in “The Conversation,” an assistant to a no-nonsense, paranoid surveillance expert played by Gene Hackman, was less tragic than Fredo and Sal. The elusive Hackman, who came out of retirement to talk about working with Cazale, discussed how Cazale pushed him as an actor.

Everyone from Pacino and Hackman to Streep and Robert De Niro praised Cazale’s commitment to acting. Pacino said he learned more about acting from Cazale than from anybody. Streep mentioned how he helped her improve her craft and added, “I think of him all the time.”

I wondered if her words had a double meaning, if she was talking about Cazale the actor, Cazale the man or both.

By the time the role as Stan in “The Deer Hunter” came along Cazale was suffering from lung cancer, and the studio refused to insure him. Streep suspects De Niro paid the bond so Cazale could make the movie.

At that point in the documentary these larger than life performers ceased being Meryl Streep, Robert De Niro and Al Pacino the actors. As they talked about Cazale’s battle against cancer, I viewed them as real people who still miss their friend.

Cazale’s brother, Steve, broke down as he recalled the day he learned of the cancer diagnosis. He also described Streep as an “angel” for taking care of his brother. Pacino said that despite Streep’s career achievements, when he thinks of her, it’s her devotion to and love for Cazale that come to mind.

Pacino usually acts like he’s being interrogated for a capital crime during interviews, but he showed a more animated side while talking about the man he thought of as a brother.

Philip Seymour Hoffman, Sam Rockwell and Steve Buscemi also proclaimed their admiration for Cazale. Buscemi said that after watching “The Godfather,” his brother compared him to Fredo. The comparison was meant as an insult, but when Buscemi saw the movie, he took it as a compliment.

The documentary’s special features include an extended interview with Pacino. When the director asked Pacino why he thinks so many people on the street identified Cazale as Fredo, Pacino explained that Cazale became the characters he played. He became Fredo, Sal, Stan and the other Stan.

Do yourself a favor, (re)discover these characters and watch the documentary.