The situation at the opening of the movie is based on an incident when, as a child, author Dennis Lehane's mother severely reprimanded him for getting into a car with men who claimed to be plain-clothes policemen.

The studio executives at Warner Brothers wanted Clint Eastwood to shoot this movie in Toronto, Ontario to save money. Eastwood refused and pushed to have this movie completely shot in Boston where it is set.

Michael Keaton was originally cast as Sean Devine. He had done several script readings with the rest of the principal cast, and he had been doing research for the role with the Massachusetts State Police Department, and living in Boston for several weeks. A month before principal photography was to start, Keaton and producer and director Clint Eastwood got into a huge argument, and Keaton left the project. A week later, Kevin Bacon was cast in the role. Keaton was in a movie about child abuse by the Catholic church in Boston: Spotlight (2015) (only as a reporter rather than a detective).

At around 5 minutes in, when a young Dave is thrown into the backseat of the car, a man in the front passenger seat turns around and displays his ring, a bishop's ring. The book never indicates that the character was a priest, but it was added to this movie since filming took place during the priest sexual abuse scandal in the Boston archdiocese.

This movie was shot in thirty-nine days.

While researching his character's occupation, Kevin Bacon had worked in the office of the Massachusetts State Police.

Laura Linney shot this concurrently with Love Actually (2003), commuting across the Atlantic for both shoots.

In Sean Penn and Tim Robbins's only previous movie together, Dead Man Walking (1995), both were respectively Oscar-nominated as Best Actor and Best Director. On this one, both were victorious as actors.

Eli Wallach claimed that he filmed his scene in one take and without a single word of direction from Clint Eastwood, who had such confidence in his acting that merely knowing his lines would be enough.

At a press conference for this movie, producer and director Clint Eastwood condemned the Iraq war as a "big mistake" and defended Sean Penn's visit to Baghdad, saying he might have done the same thing, but for his age.

Director of the Cannes Film Festival Thierry Frémaux often said that if he had one bad memory of the festival, it was to call Clint Eastwood to tell him that this movie did not win any award.

Forest Whitaker was the original choice for Whitey Powers and almost accepted, but backed out supposedly due to other commitments.

Clint Eastwood narrated the teaser trailer.

The movie Dave Boyle is watching is Vampires (1998), starring James Woods.

More than two thousand people who wanted to be extras in this movie showed up at the Boston Teachers Union on September 14, 2002 for the open casting call. Carolyn Pickman of Collinge-Pickman Casting told The Boston Globe a week later that about two-thirds of those trying out would be called to fill a church and a local bar, play spectators at a parade, or walk through other scenes. All types of people from the greater Boston area (and as far away as New Hampshire) attended the call, where they waited in long lines to eventually hand in their applications and get their pictures taken.

The cast includes three Oscar winners: Sean Penn, Marcia Gay Harden, and Tim Robbins; and two Oscar nominees: Laurence Fishburne and Laura Linney.

(At around one hour and fifteen minutes) The liquor store scene was shot at Costello's, a liquor store on Boylston Street in the Back Bay, next to Berklee College of Music. After filming, screen shots from that scene were placed in the front display windows. Costello's closed in January 2007.

In Dennis Lehane's novel, the surname of the characters of Jimmy and his family is "Marcus". However, in this movie, it is "Markum".

Kevin Bacon and Laurence Fishburne portrayed Massachusetts State Police partners Devine and Powers in this movie. They first appeared together as rival bicycle messengers Casey and VooDoo in Quicksilver (1986).

Marcia Gay Harden was the only Best Actress in a Supporting Role Oscar nominee that year that was from a Best Picture nominated movie.

The tune the band is playing during the parade at the end of the movie is John Philip Sousa's "Semper Fidelis", Latin for "always faithful".

Sean Penn and Tim Robbins, were Clint Eastwood's choices for the leads from the outset.

Laura Linney (Annabeth Markum) is the only major cast member who does not cuss in this movie.

The only movie that year to be nominated for Best Actor in a Supporting Role and Best Actress in a Supporting Role Oscars.

The tombstone scene was shot in Roslindale at Wellsmere Monumental Works, as it had the background of an overground field on two sides of it and on the other side an old 1820's house that still stands at 3849 Washington Street.

The only Best Picture Oscar nominee of 2003 to be nominated for Best Actor in a Supporting Role (Tim Robbins) and Best Actress in a Supporting Role (Marcia Gay Harden). Tim Robbins won the Oscar for his performance.

Kevin Bacon appeared with Sean Penn's brother Chris Penn, in Footloose (1984).

Second movie featuring Marcia Gay Harden directed by Clint Eastwood. The other being Space Cowboys (2000).

Tom Guiry and Kevin Conway appeared on The Black Donnellys (2007).

Laura Linney and Marcia Gay Harden appeared in P.S. (2004).

Eli Wallach: (At around one hour and fifteen minutes) He appeared uncredited as a liquor store owner in his first movie with Clint Eastwood since The Good, the Bad and the Ugly (1966).

Dennis Lehane: (At around two hours and ten minutes) The author of the novel can be seen waving from the back of a convertible in the parade sequence.

(At around 33 minutes) For the scene where Jimmy Markum wails and thrashes in anguish at the discovery of Katie's body, Sean Penn requested that a tank of oxygen be standing by after he finished the take.

(At around one hour and six minutes) During the scene where Jimmy Markum steps into the morgue where his daughter Katie is lying on the slate dead, and when he is emotionally promising revenge, Emmy Rossum burst into tears, saying that "the scene was so powerful and moving, and Sean Penn was so amazing."

Body count: three.