Because of the way the movie was shot, Mickey Rourke (Marv) and Elijah Wood (Kevin) never met until after the film was released.
The swords used by Miho (Devon Aoki) in this movie are the same ones used by some of the Crazy 88 in Kill Bill: Vol. 1 (2003). That movie's director, Quentin Tarantino, had been keeping them in the back of his garage.
Robert Rodriguez has said that he does not consider this movie to be an adaptation so much as a translation. This is why there is no screenwriting in the credits. The only mention of writing is Frank Miller as the creator of the graphic novels.
Originally, Robert Rodriguez didn't plan for Benicio Del Toro to wear make-up, but Del Toro insisted on it. Tarantino later commended the make-up being so good that "people actually forget that's not what Benicio looks like."
Guest director Quentin Tarantino directed the scene involving Dwight (Clive Owen) and Jackie Boy (Benicio Del Toro) in the front of the car before Dwight is pulled over by a police officer.
Frank Miller: The co-director and creator of the original comic series has a cameo as a priest. (This would not be considered as a cameo. Those are only for UNcredited actors, and the credits list Frank Miller as the Priest.)
Robert Rodriguez: [Chango Beer] When Hartigan (Bruce Willis) comes into Kadie's Bar, Shellie (Brittany Murphy) is carrying a bottle of "Chango Beer." This is the same fictional brand used in other Robert Rodriguez movies, including Desperado (1995) and From Dusk Till Dawn (1996).