After he decided to make the film, Al Pacino invited Frank Serpico to stay with him at a house that Pacino had rented in Montauk, New York. When Pacino asked Serpico, "Why did you do it?" Serpico replied, "Well, Al, I don't know. I guess I would have to say it would be because... if I didn't, who would I be when I listened to a piece of music?"
After spending a lot of time with Al Pacino as he prepared for the role, the real Serpico wanted to remain on the set during filming. Producer Martin Bregman said he had to hurt Serpico's feelings and ordered him to leave because he believed that his presence would prove a distraction.
The film ends with the words on the screen saying that Serpico is living somewhere in Switzerland. In fact, Frank Serpico discovered he didn't like Switzerland and returned to the U.S. As of 2015, he was living on a small farm in upstate N.Y., and working as a guest-lecturer at police academies across the United States.
Playwright Sidney Kingsley loaned his apartment to Sidney Lumet for use to film the party scene. In 1935, Kingsley hired an 11-year-old Lumet to appear on Broadway in his play, "Dead End", and they had remained friends since then.
The large cruise ship that is behind Serpico in the end credits is the S.S. France, which later was purchased by Norwiegian Cruise lines and re-named the S.S. Norway. She was first launched in 1960, and continued her career until about 2006, when she was sold, anchored off the coast of India, and slowly taken apart for scrap.
According to Sidney Lumet, Al Pacino always needed to be in the character's state of mind in any given scene and could not shed that state off camera, so he behaved accordingly at all times, either happy, joking, and laughing for a lighthearted scene or angry and lashing out at everyone if the scene they were working on called for that behavior.
When Serpico's next-door neighbor Laurie (Barbara Eda-Young) sees him listening to opera music in his garden she asks, "Is that Björling?" Frank says, "It's Di Stefano." Both references were to operatic tenors Jussi Björling and Giuseppe Di Stefano, legendary singers from Sweden and Italy.
Woodie King Jr. was originally cast as a hoodlum but broke his leg while filming a chase scene. He was replaced but returned to the set two months later to play Leslie's friend, Larry, in the party scene.
Sidney Lumet was pleased with the cooperation of the NYPD, especially in light of the subject matter and the proximity in time to the actual events depicted in the movie. Two officers were directly assigned to the movie, and Lumet wondered what their reaction would be. "As soon as they saw the truth we were going for, how it was not a Hollywood version, they not only weren't a problem, they more actively helped," he noted.
Before shooting began, Sidney Lumet and the production company had to cast 107 speaking parts. They decided to use mostly unknown actors. Lumet said the best way to strengthen the sense of reality was not to use actors for whom audiences had a lot of previous associations. Even Al Pacino, despite his high exposure in The Godfather (1972), was still relatively new.
Speaking of the production in a later interview, Sidney Lumet said, "Serpico, just physically and in terms of logistics, gives you the problem of keeping your emotional theme work in perspective. You have to ask yourself not only 'Where am I physically?' but 'Where am I emotionally?' I think I was more tired after finishing Serpico than almost any movie I've ever done. There was also the obligation to the real Frank Serpico--to be honest with his life and not exploit it."
Al Pacino would sometimes go in character to different neighborhoods, some of them dangerous. One story has it that Pacino was so in character that he pulled over a truck driver and threatened to arrest him for exhaust pollution.
Sidney Lumet liked to do very simple things on the first day of shooting, like basic entrances and exits, to let actors and crew get used to each other and make them aware that things will move very quickly. He will often shoot just a single take and move quickly to another set-up. He said this process also helps to spot weak links in his team. The first day on this film, he worked at three different, fairly far-flung locations. Al Pacino was initially stunned, especially after coming off the methodically low, deliberate process of The Godfather (1972). But he and the rest of the cast soon learned that this fast pace had the benefit of keeping the inner tension of the narrative and the characters alive.
The film was scheduled to open by Christmas. That left four and a half months for shooting, editing, and mixing, an "insanely short time" in Sidney Lumet's estimation. Therefore, the editing had to take place during filming. Without the luxury of time, it was necessary to finish shooting a scene and rush it to editor Dede Allen, who had to cut the footage within 48 hours and have it ready for delivery to the sound department.