Underworld settings in the USA featured in the movie included Skid Row in Los Angeles and Little Italy and Spanish Harlem in New York City.
The unsettling poster Bronson's character has in his apartment is a detail from "Saturn Devouring His Son" a painting by Spanish artist Francisco Goya.
In the novel, the main character is called Derek Torry, not Lou, and the entire story is set in the UK.
A "stone killer" is an underworld term for a professional hitman who kills with no remorse or conscience.
John Ritter and Charles Bronson both passed in 2003 almost two weeks apart. Bronson died on August 30th, while Ritter passed away on September 11th.