Mark Wahlberg based his performance on the police officers who'd arrested him about two dozen times in his youth, and the reactions of his parents who had to come bail him out with their grocery money.
Throughout the film, Martin Scorsese used Xs mostly shown in the background to mark characters for death; examples include shots of Costigan walking through the airport while talking to Sgt. Dignam, Queenan falling to his death (on the building's glass windows as Queenan falls to the ground), and Sullivan in his office discussing the flow of information with Costello (the X is created by the light shining through the window). This is a homage to Howard Hawks' classic film Scarface (1932).
The scene where Frank Costello throws cocaine on hookers was one of many bizarre ideas contributed by Jack Nicholson, who also suggested wearing a strap-on for the scene with Matt Damon in the porn theater.
A possible reason why Leonardo DiCaprio did not receive an Oscar nomination for his performance in this movie was because the Warner Bros. Studios initially did not want to favor DiCaprio over his co-stars and place him in the leading actor category. The studio favored DiCaprio's leading performance in Blood Diamond (2006) (which eventually got him a nomination). DiCaprio himself refused to campaign against his male co-stars in the supporting actor category, so Warner bought no supporting actor ads for DiCaprio, and he did not receive a nomination.
Originally, Jack Nicholson turned down his role in the movie, but after a meeting with Martin Scorsese, William Monahan and Leonardo DiCaprio, he was finally convinced to play the role of Frank Costello. The main reason he joined the production was because he had previously done a few comedies, and wanted to play a villain again, and he considered the character of Costello to be the ultimate incarnation of evil.
Martin Scorsese wanted to shoot the film in Boston, where the story is set. But due to concerns on setting up production and politics, the producers chose New York City to double for Boston because of the state's 15% tax credit. The bulk of the movie was shot in New York City while a six-week shooting schedule was split in two for Boston, shooting the first half in June and the second half in August. After the success of this film, Massachusetts created a 25% tax credit for filmmaking.
After completing The Aviator (2004), Martin Scorsese kept Alec Baldwin in mind for future collaboration and ultimately decided to cast him in the role of Ellerby which was offered to Mel Gibson first, but Gibson was unable to accept the part because he was starting production on Apocalypto (2006) at the time.
As research for his character's occupation, Matt Damon worked with a Massachusetts State Police unit out of Boston. He accompanied them on routine patrols, participated in a drug raid and was taught proper police procedures like how to pat down a suspect.
When Queenan and Dignam are interviewing Costigan, Costigan says "Families are always rising and falling in America." Queenan wants to know who said that, and it turns out to be Nathaniel Hawthorne. Dignam quips, "What's the matter smart ass, don't know any fuckin' Shakespeare?" Later, as Queenan hands the clipboard to Sullivan, it is Queenan who quotes William Shakespeare with "the readiness is all," from Hamlet's "Fall of a sparrow speech," Act V, scene ii.
When the main characters are shown in a police academy ballistics lecture at the beginning, the large flip chart illustrations seen in the background are Warren Commission exhibits of President John F. Kennedy's head wounds, prepared by medical illustrator H.A. Rydberg under the direction of Dr. James Humes, the chief examiner of Kennedy's autopsy.
Colin Sullivan's (Matt Damon) apartment does not exist. The view of the Massachusetts State House was an effect shot from the roof of Suffolk University, which is the law school where Sullivan says he is taking night classes. Michael Ballhaus, the film's cinematographer, evaluated the shot during preproduction.
The CD that Costigan mails to Colin is mailed in the cover for The Rolling Stones' album "Exile on Main St.". Earlier in the film, when Costello beats Costigan's hand with his own shoe, a song from the album, "Let It Loose", plays over the scene.
The first Best Picture Oscar-winner of the 21st century that wasn't released on VHS in the United States, and the first to be released on the short-lived HD-DVD format. Warner Bros. Home Entertainment had already phased out VHS by 2006, therefore, the film was initially released on DVD, Blu-ray, and HD-DVD the following year.
In the dinner scene with Madolyn, Colin states that "what Freud said about the Irish is we're the only people impervious to psychoanalysis." Despite what you may find on a Google search or the Boston Globe, Sigmund Freud didn't actually ever say that. In a clever act of investigative journalism, a man named Dr. Charles wrote to the director of research at the Freud Museum in London, and asked him about the legitimacy of the quote's attribution. His response (which is also stated on the FAQ section of the museum's Website): "There is no evidence Freud said [the quote]. The only documentation seems to be Anthony Burgess, in his introduction to a book of Irish short stories: 'One of [Freud's] followers split up human psychology into two categories - Irish and non-Irish.'"
The film's technical advisor was Thomas B. Duffy, a retired Massachusetts State Police major who worked out of Boston for nearly thirty years and specialized in organized crime. He was particularly involved in the case against notorious South Boston mob boss James 'Whitey' Bulger, whom Frank Costello is partly based on. Duffy appears as the Governor who delivers a speech to the graduating police cadets. There was an unconfirmed sighting of Bulger, one of the FBI's Ten Most Wanted, at a theater showing the film by a deputy sheriff in San Diego, California. Bulger would be captured in Santa Monica, California on June 22, 2011; he'd been living in an apartment complex just a few blocks away from the production offices of 'GK Films [us]', who produced Edge of Darkness (2010) which Duffy also appeared in.
During the exchange with the Chinese gangsters, Sullivan sends a text message to Costello saying that all cell phone calls are being monitored. The number dialed by Sullivan is actually a real Boston area code (617).
The "MASS Processor Company's" microprocessor shown in the movie is really an ST Microelectronics' ST9F150JDV1QC micro-controller, released in 2003 and intended for applications such as MP3 players, GPS devices, and car radios. It went for around $7 at the time it was released (in the movie it is stated they go for $100,000 each). The microprocessor has an internal clock frequency of 24 Megahertz and 100 pins and can address up to 4 Megabytes of memory. A typical Intel Core 2 Duo microprocessor, released in 2006 (the same year as The Departed) for use in desktop computers, runs at 1400 Megahertz or higher frequencies, has 775 pins, and can address up to 4000 Megabytes.
The newscaster seen reporting the news story detailing the dumped body by Costello's gang was a real Boston area newscaster at the time of filming. He reported for Boston's Warner Bros. affiliate station WB56.
The classroom scene, police academy graduation scene, and shooting range scene (all at the beginning of the film) were actually shot near the end of production. Historic Ft. Schuyler on the campus of State University of New York's Maritime College was the back drop.
The trivia items below may give away important plot points.
At the beginning of the film Frank Costello instructs the store clerk to fill a white paper bag with various groceries for the kid Colin Sullivan, notably a couple of loaves of bread and a couple of quarts of milk. In the last scene of the film we see adult Colin Sullivan walking into his apartment with a white paper bag full of groceries, two of the items you can see in the bag during this scene are a couple of loaves of bread and a couple of quarts of milk.
Many scenes with Jack Nicholson were improvised. Nicholson was given the opportunity to do whatever he wanted to add to the character's unpredictability. The scene where Billy and Frank are talking was loosely scripted, and many surprises happened in it, including Frank pulling out the gun.
Right after the time skip in the beginning of the movie, during the first lesson at the police academy, the teacher is elaborating on the details of a gunshot wound to the head, which is the leading cause of the vast majority of deaths throughout the course of the rest of the movie.