Although he had initially agreed to play the part of Sonny, Al Pacino told Sidney Lumet near the start of production that he couldn't play it. Pacino had just completed production on The Godfather Part II (1974) and was physically exhausted and depressed after the shoot. With his reliance on the Method, Pacino didn't relish the thought of working himself up to a state of near hysteria every day. Lumet unhappily accepted the actor's decision and dispatched the script to Dustin Hoffman. Pacino changed his mind when he heard that his rival was being considered.
During production, Al Pacino reportedly only slept a couple hours a night, ate sparingly, and would sometimes take cold showers; this was in order to emphasize Sonny's disheveled, exhausted and yet wired appearance.
Another notable improvisation in the film was John Cazale's answer to Al Pacino's question of where in the world he'd like to fly to. Pacino's surprised response was absolutely genuine as he had no idea what Cazale was going to say.
The real bank robber, John Wojtowicz, had watched The Godfather (1972) to get ideas the day he robbed the Chase Manhattan bank. Al Pacino stars in both that film and this one.
Much of the film's dialogue is improvised, while completely honoring the structure of the original screenplay. After rehearsing for three weeks with his cast, Sidney Lumet took the improvisations from those rehearsals and made that the official screenplay.