Dawson Casting: Cazale was in his 30s when he played Sal, who in Real Life was barely out of his teens. Lumet didn't want to cast Cazale at first due to the obvious age discrepancy, but Cazale gave too good a performance.
Throw It In: Much of the dialogue was ad-libbed or improvised, most notably "ATTICA! ATTICA!" Al Pacino also didn't know that John Cazale was going to say "...Wyoming?" when asked what foreign country he wanted to escape to, and his reaction is genuine.
Unintentional Period Piece: Now that the Attica riots have largely faded from popular memory, Sonny using the name as a shorthand for police brutality places it firmly in the '70s. Especially telling is that a spoof of the scene in Airheads replaces it with a chant of Rodney King.