The character of "Bubbles", played by Andre Royo, was largely based on a real Baltimore drug addict and police informant who went by that name. The real Bubbles, who would slur his speech much more than the fictional character, first started working with the cops near the early 1960's after being arrested for a burglary. He spent the next few decades as an informant, leading to the arrest of several hundred felons and getting paid at least $50 for each one caught. His photographic memory and ability to blend into the scene made him one of the best in the city. Eventually he would start using the hat method portrayed on the series, where he acted as if he was selling hats and would place one of a certain color on the head of those the police should arrest.
Many of the minor characters are played by real-life police officers, politicians and former criminals. In fact, many of the former criminals who act on the show were previously arrested by the real-life cops who act on the show.
When Dominic West first auditioned on videotape from his London home, he tried to have his girlfriend read the lines for the other characters in the scene. But her English accent kept throwing him off and he kept laughing. So West performed the scene himself by leaving pauses where the other character's lines were supposed to be. West admits to imitating Robert De Niro for his audition. At first, the producers found the audition tape "weird" and "comic" but they reconsidered when they concentrated on West's performance. When West was offered the role, he became reluctant because the contract was for five seasons. But his agent convinced him that the show would not last more than one. It ended up lasting five seasons.
In season three, when McNulty is comparing the units skills favorably to those of other Baltimore police he mentions Donald Worden as one of the few police who can match them in skill. Worden is a real life detective who served in the Baltimore homicide unit when David Simon covered it. For at least part of that time he was the partner of Jay Landsman, who plays Dennis Mello and was the inspiration for the character of the same name.
According to Sonja Sohn, during the first season she had much trouble remembering her lines causing numerous delays. She says it was due to childhood trauma of growing up in a ghetto and witnessing police brutality. Because of this she was uncomfortable in the neighborhood while filming took place and had issues with portraying a police officer.
According to Michael K. Williams, he secretly struggled with a cocaine addiction during the third season. He never missed a day of work nor was he ever late. He also suffered with an identity crisis due to his popularity as Omar.
Tray Chaney originally auditioned to play the character of Wee-Bey Brice but the producers felt he was too short. They were so impressed with his audition that they created the character of Poot for him to play.
In season one, two detectives, Augustus Polk and Patrick Mahon - were assigned to the Barksdale detail. The names "Polk" and "Mahon" are a play on "pogue mahone," the anglicized version of an Irish expression meaning "kiss my ass."
In season four Dominic West, the ostensible star of the series, requested a reduced role so that he could spend more time with his family in London. On the show it was explained that Jimmy McNulty had taken a patrol job which required less strenuous work.
Some of the show's interiors, including McNulty's apartment set and the police station's offices, were constructed inside a former Sam's Club Warehouse store. The zoning for that building dictated that its tenant had to be a retail business, and when the producers got into trouble for renting the building without selling anything from it, they considered opening a store there to sell t-shirts, DVDs, and other show souvenirs before the issue was finally resolved.
Much like its sister show Homicide: Life on the Street (1993), contains numerous references to David Simon's book "Homicide: A Year On The Killing Streets". For instance, the opening of the very first show is lifted directly from the book.
The character of Sgt. Jay Landsman played by Delaney Williams was based on Jay Landsman, a real-life Sergeant with the Baltimore County Police. He was featured in David Simon's book "Homicide: A Year On The Killing Streets". Landsman himself joined the cast in the third season playing Lt. Mello.
David J. Smolar, who appears in the final episode of season 3 as a reporter talking to Councilman Gray, previously appeared as the piano player in a restaurant scene between D'Angelo and his girlfriend in episode 5 of season 1. Smolar performed the piano music heard in that episode, a posthumous Chopin Etude and later movement 2 of Beethoven's Pathetique Sonata.
The writers/producers briefly considered doing a sixth season about the influx of Latinos into Baltimore. But none of them knew enough about Baltimore's Latino population to write about it so the idea was dropped.
In an unusual move for a police series the officers almost never fire their guns. Only one police officer, Prez, ever fires his gun on the show and in all three cases it is a mistake. The first time he accidentally shoots the squad room wall. The second time he drunkenly fires a shot into the air and then (compounding his gun mistakes) uses it to pistol whip a teenager, blinding him in one eye. The third time he mistakenly shoots an undercover cop.
Producer Robert F. Colesberry also plays Detective Ray Cole. After Colesberry died tragically following routine heart surgery, the Ray Cole character also died from a heart related illness in the show. The entire cast participated in an elaborate detectives wake in honor of Cole and Colesberry's memory.
Although Cheese Wagstaff and Randy Wagstaff have the same last names, it is never stated that they are related. David Simon has since confirmed in interviews that Cheese is Randy's father. Simon planned to reveal this in a fifth season episode but later decided against it.