John Cazale spent over a decade in theater before The Godfather gave him his first film role. He played Fredo Corleone in 1972, a character defined by weakness rather than villainy, which made him more unsettling than the mob bosses surrounding him. He followed that with The Conversation, The Godfather Part II, and Dog Day Afternoon, working with the same tight circle of collaborators. Every film he ever made was nominated for Best Picture. Three of them won.
Cazale was diagnosed with lung cancer in 1977 and kept working. When he became effectively uninsurable for The Deer Hunter, Robert De Niro reportedly covered the insurance costs to keep him on the film. Meryl Streep, his girlfriend, took a role she described as a 'vague, stock girlfriend' specifically to stay near him during production. Michael Cimino rearranged the entire shooting schedule so Cazale could film his scenes first. He finished. He died over a year before the film won Best Picture.
He never had a lead role. He played supporting parts in films built around Brando, Pacino, Hackman, and De Niro, and still managed to be the one people remember. His friendship with Pacino went back to their theater days in Boston and New York. Casting director Fred Roos spotted him and recommended him to Coppola for The Godfather. A 2009 documentary, I Knew It Was You, brought him back into public conversation 30 years after his death. Streep, Pacino, De Niro, and Coppola all showed up to talk about him.
The Deer Hunter won Best Picture at the 51st Academy Awards in April 1979, over a year after Cazale's death. His director Michael Cimino refused to participate in the 2009 documentary I Knew It Was You: Rediscovering John Cazale, directed by Richard Shepard, which premiered at Sundance. All five of his films have been selected for preservation in the Library of Congress National Film Registry.