Throughout the seasons, the characters can be heard talking about a firefighter name Paddy Brown and the different things he did. Paddy Brown was a real life firefighter who gave his life on 11 September 2001.
Throughout the show, Tommy is seen wearing a "Leary Firefighters Foundation" t-shirt. The Leary Firefighters Foundation is a charity which was founded by Denis Leary in response to the 1999 fire in Leary's hometown of Worcester, Massachusetts, which killed six firefighters, one of them being Leary's cousin. The LFF provides funding and equipment for fire departments in the United States.
Lenny Clarke was originally offered the part of Chief Reilly, but turned down the role due to his commitment with It's All Relative (2003) and took up the part of Gavin's Uncle Teddy after that show was cancelled.
In the pilot, Franco (Daniel Sunjata) jokes that he will get back at someone by reporting that he "touched me in the shower". Sunjata was nominated for a 2003 Tony Award for Best Featured Actor for the play "Take Me Out", in which his character memorably sexually assaulted another man in a locker room shower.
Adam Ferrara, a good friend of Denis Leary's and who co-starred with him on Leary's previous television show The Job (2001), was approached early on for a role in the show. He initially couldn't commit due to other engagements but did have a large role later on as Chief 'Needles' Nelson.
In episode two, "Gay", Tommy Gavin (Denis Leary) is on the phone with his brother, John Gavin. John, using a cell phone, passes a promotional poster for Shorties Watchin' Shorties (2004), which Leary produced and which features his voice.