Amy Ryan looked and sounded so convincing as a low class Dorchester mom, that a security guard mistook her for a fan on the first day of location filming, and wouldn't let her on the set. One of the producers finally noticed her on the other side of one of the barricades, and said she should be let through. The incident made Ryan twenty minutes late, but convinced her the Boston accent she'd prepared was realistic.
Jill Quigg was discovered when the production's barricades in the streets of Boston prevented her from picking up her son from school. Ben Affleck was so impressed by her loud rants against the film crew, that he asked her to read for the movie.
The film's UK release, scheduled for December 28, 2007, was postponed for almost six months, because of the film's similarity to the real-life case of four-year-old Madeleine McCann, who disappeared from the holiday apartment, where she and her family were staying in Praia da Luz, Portugal, on May 3, 2007.
On the DVD commentary, Ben Affleck says that he and Jill Quigg improvised the scene early in the movie in which Quigg (as Dottie) talks to the press about the flyers they had posted and the vigils they had planned. Affleck says that he would feed her some lines and then she would perform them. Her mispronunciation of the word "vigil" as "visual" came from her mishearing what Affleck had said from behind the camera, but he later decided to keep it the way she had said it.
The character of Jerome Miller (a.k.a. Bubba, Patrick's drug-lord friend) is played by Boston hip-hop performer Slaine. In the scene where Bubba and Patrick are driving up to Everett to meet Leon and Roberta Trett, the song playing on the stereo in their SUV is by Slaine.
During the middle of filming, Michelle Monaghan also started work on The Heartbreak Kid (2007), for the Farrelly brothers, which required her having her bangs cut. These had to be added as a visual effect to maintain continuity.
In the novel, "Cheese" is Sven "Cheese" Olamon, an imprisoned gang leader of Scandinavian descent, who went to high school with Patrick and Angie. In the film, "Cheese" is Cheese Jean-Baptiste, a Haitian criminal.
Helene (Amy Ryan) says she did a drug run to Nashua while being interrogated by Remy Bressant (Ed Harris) in her apartment. Amy Ryan's character, Holly, in The Office (2005), is transferred to the Nashua office after her relationship to Michael Scott is discovered.
The trivia items below may give away important plot points.
The cartoon playing in the background at the end of the film is in the public domain, as no cartoon creators would let the production use their footage in such a controversial scene. Likewise, the "RoboDudes" brand, and children's t-shirt worn by Corwin Earle, had to be created by the film crew, since no toy or cartoon company would allow a representation of their product to be worn by a pedophile child murderer character.
When Patrick & Angie arrive at Helene's place to make initial enquiry then the TV flashes a scene with Uncle Pimp & Cupid. A subtle hint towards Lionel. Also the Restaurant where the Lionel confesses is called Murphy's law and things do go wrong for all concerned.