The film was shot in reverse order. Al Pacino began with long hair and a beard, then for each scene, his hair and beard were trimmed bit by bit until he became clean-cut.

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, NY. 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?"

The film ends with an epilogue that says, "Frank Serpico resigned from the Police Department on June 15,1972. He was awarded the Medal of Honor for conspicuous bravery in action. Serpico is now living somewhere in Switzerland." Serpico discovered he didn't like Switzerland, and returned to the US. As of 2015 he was living on a small farm in upstate New York and working as a guest lecturer at police academies across the US.

Al Pacino considers this movie to be one of his greatest achievements as an actor.

After spending a lot of time with Al Pacino as he prepared for the role, the real Frank 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.

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.

The film was scheduled to open by Christmas. That left 4-1/2 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.

Principal photography began a year after Frank Serpico resigned from the New York City Police Dept.

Associate Producer Roger Rothstein gave Sidney Lumet high marks as "a tremendously organized director" who was able to motivate everyone to do as many as 35 set-ups in a single day.

The film was shot at 104 different locations in every borough of New York City except Staten Island.

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.

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.

The film was first planned as a star vehicle for Paul Newman and Robert Redford, following their success in Butch Cassidy and the Sundance Kid (1969). Redford was to play Frank Serpico and Newman was to play his lawyer friend David Durk (a character renamed "Bob Blair" in the final film, where he is played by Tony Roberts).

In the film, Serpico has two girlfriends, Leslie and Laurie. In real life, he married both women, plus two more, Mary Ann and Marianne.

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.

The actors were allowed to do some improvisation in their scenes. Much of Al Pacino's explosive reaction in Serpico's last abortive meeting with his former captain was off the cuff.

The aria Serpico listens to in his garden is "E lucevan le stelle" ("And the stars were shining") from Giacomo Puccini's opera "Tosca". Among other things, the opera deals with corruption and abuse of power.

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.

John G. Avildsen was originally to direct, but was replaced just before filming due to differences with the producer.

In the movie, Al Pacino lives at 5-7 Minetta Street in Greenwich Village. In real life, Frank Serpico lived at Perry and Greenwich, a few blocks away.

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".

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.

Before filming, Al Pacino spent a great deal of time with the real Frank Serpico to perfect his performance.

Since filming was done in the summer heat, details of winter scenes, such as defoliated trees and visible breath, had to be simulated.

The film takes place from 1960-72.

The production budget was in the $2.5-$3 million range.

Frank Serpico's nickname "Paco" is actually "Francis" in Spanish. "Paco" is short for "Francisco".

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.

When Serpico's next-door neighbor Laurie 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.

The film cast includes two Oscar winners, Al Pacino and F. Murray Abraham, and one Oscar nominee, Judd Hirsch.

According to director Sidney Lumet, filming began on the July 4, 1973.

Included among the "1001 Movies You Must See Before You Die", edited by Steven Schneider.

The large cruise ship behind Serpico in the end credits is the S.S. France, which later was purchased by Norwegian 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.

Film debut of Barbara Eda-Young.

All (or nearly all) of the new police officers reporting for their first day at work are wearing plaid shirts.

Only film credit of Norman Ornellas.