In the comics, Dylan Dog's sidekick is a Groucho Marx impersonator who goes by the name Groucho and behaves exactly like the comedian, having lost all recollection of his true identity. This had to be changed for the film, due to legal issues related to the use of Groucho's likeness.
In a photograph in Dylan's flat, we can see him and his assistant masked as Groucho Marx. In another scene, we can see that Dylan keeps his weapons stashed in a safe behind a Soupe au canard (1933) poster (a Marx brothers movie). Both are references to the Dylan Dog comics on which this movie is loosely based on. Although in the comics, Dylan's assistant, sidekick and comical relief is not the zombie from the movie, but a stage actor who looks exactly like Groucho Marx, calls himself Groucho, and always dresses and behaves like the legendary comedian (it's implied this is due to the character's slight insanity).
This is the second film Brandon Routh and Sam Huntington have collaborated on, with the first being Superman Returns (2006).
When held in the vault, Sam Huntington calls Brandon Routh "Superman". Huntington and Routh starred as Jimmy Olson and Clark Kent / Superman, resp., in Superman Returns (2006).