The name John Cazale doesn’t mean much to most people. If they do remember him, they might think of the odd-looking guy in The Godfather and Dog Day Afternoon. But Cazale is a patron saint to Al Pacino, Meryl Streep, Robert De Niro and the generation of character actors who came after him.

The reasons for his exalted status are as entangled with nostalgia for a lost era in filmmaking as they are with respect for the brilliant performances this shape-shifting actor gave during his short tenure as a movie star. Cazale made just five films before he died of lung cancer in 1978 at 42. But they happened to be five of the greatest films of the century, each nominated for Best Picture Oscars, two of them winners. He was the heartbreakingly weak Fredo Corleone in The Godfather and its sequel, the trigger-ready Sal in Dog Day Afternoon, Gene Hackman’s dogged assistant Stan in The Conversation and the goofy Stanley in The Deer Hunter.

Meryl Streep had never spoken so publicly about Cazale. And if her emotional interview in the film is any indication, it wasn’t especially easy.

Though Cazale never won an Oscar nomination himself, the actors playing opposite him did. And they all credit their early success with Cazale’s idiosyncratic work style. He knew how to improvise, how to lead a fellow actor into a state of mind, how to goad them into their best performances. A new HBO documentary, I Knew It Was You: Rediscovering John Cazale, debuting Tuesday, erases any doubt of Cazale’s influence. Indeed, it’s evident that he helped groom the young actors who went on to become de-facto sovereigns of Hollywood.

“I learned more about acting from John than anybody,” Pacino says in the film.

The documentary is short on details about Cazale’s life. And there’s no footage of him speaking out of character. But it is compelling to see the emotion this relatively obscure man with the vast receding hairline and pain-filled eyes inspires among such high caliber talent.

“John’s life was such a mystery,” said the documentary’s director-producer Richard Shepard. “He liked to drink. He liked to smoke. He liked beautiful women and he liked to act. His personal story was what it was. Once Meryl Streep said she’d learned about acting from John and Pacino said the same thing, I realized that, ultimately, this movie would be about acting in the guise of—John.”

Cazale was born in 1935 in Revere, Massachusetts, the middle child of three with a father who sold wholesale coal. He left home to study theater at Boston University and then moved to New York to become an actor, working odd jobs and landing stage roles. He met Pacino when the two young men were Standard Oil delivery boys and they became friends while in the play The Indian Wants the Bronx. Pacino played a young street punk who attacks Cazale’s newly emigrated Indian. The play and both actors earned Obie Awards that year.

Cazale got his break in film when Godfather producer Fred Roos saw him in the 1971 revival of Line co-starring Richard Dreyfuss. “There was no hesitation to cast him,” says Francis Ford Coppola in the documentary. In fact, Coppola was so moved by Cazale’s Godfather performance, he wrote a part for him in the now-classic thriller The Conversation.

Cazale had a way of getting under the skin of other artists. In the documentary, even Hackman, notoriously single-minded about his craft, sounded a bit intimidated by Cazale’s talent, calling him “extremely intense” and “highly concentrated.” And when Cazale’s terminal illness prevented him from getting insured for The Deer Hunter, De Niro paid his way. (Cazale finished the film, but died before its release.)

Streep was in love with Cazale and they dated up until his death. And though she’s been married to sculptor Don Gummer for 31 years, Streep still wells up with emotion when she remembers Cazale’s impact on her life.

In fact, it weren’t for Streep, this documentary probably wouldn’t have been made. Shepard and producer Stacey Reiss spent several unsuccessful years trying to persuade the actress to participate. (Shepard is new to the doc genre; he has mostly worked in television, and won an Emmy for directing the pilot of ABC’s Ugly Betty.) They hoped that she could move the rest of that 1970s filmmaking clique to join. Finally, Cazale’s brother Steven cornered Streep at a Manhattan art opening. “She said, ‘I haven’t even heard about it!’” Shepard recalled with a biting laugh, “‘I’d love to!’”

Streep had never spoken so publicly about Cazale. And if her emotional interview in the film is any indication, it wasn’t especially easy. Streep was at Cazale’s bedside as he lay dying. During the film, her eyes are sometimes red and slightly swollen as if she’d broken down, recovered and soldiered on. But here she is, the most Oscar-nominated actress in history, humbly crediting Cazale for inspiring her career.

“We would talk about the [acting] process endlessly and he was monomaniacal about the work,” she says in the film. “I think I was more glib and ready to pick the first idea that came to me. He would say, ‘There are a lot of other possibilities.’ That was a real lesson. I took that to heart. I always think about it.”

Plus: Check out more of the latest entertainment, fashion, and culture coverage on Sexy Beast—photos, videos, features, and Tweets.

Gina Piccalo spent a decade at the Los Angeles Times covering Hollywood. She's now a contributing writer for Los Angeles Magazine and her work has appeared in Elle, More and Emmy. She can be found at ginapiccalo.com.