John Cazale, Fredo from the Godfather movies.
He appeared in only five feature films in his career, and all of them were nominated for Best Picture.
He was friends with Al Pacino since they were teenagers. Al Pacino, referred to Cazale as his "acting partner", the guy he could've acted with his entire life.
He won lots of Off-Broadway Obie Awards before making it into movies. Pacino invited him to audition for the role of Fredo. Unlike most of the casting of the film, he easily won his the part in thanks to a good audition and little competition for the role.
Francis Ford Coppola was impressed with his performance and wrote a much bigger and central role for the sequel, in which shares the screen for a classic scene in American cinema.
Friend Al Pacino requested him to audition for the role of Sal in Dog Day Afternoon (1975). Director Sidney Lumet was unsure, with Cazale not being anything like the real-life counterpart (the real Sal was eighteen, while Cazale was in his late thirties). Cazale immediately won over the part with the audition.
His final film, The Deer Hunter (1978), was filmed while he was ill with bone cancer. He became engaged to his co-star, Meryl Streep, whom he met when they both appeared in the New York Public Theater's 1976 production of Shakespeare's Measure for Measure.
The studio was unaware of his condition, but the director knew. Cazale was weak and needed to film his scenes first. When the studio discovered he was suffering from bone cancer, they wanted him removed from the film. His co-star and fiancé, Meryl Streep, threatened to quit if he was fired. He died shortly after filming was completed.