Rhona Mitra auditioned for the role of Selene in Underworld (2003) but lost it to Kate Beckinsale. For this film, when Mitra was thought of as an option, concerns were brought up that there was too much of a physical resemblance between the actresses. Those concerns were quickly dropped when it was mentioned that the very reason Viktor turned Selene into a vampire was due to her resemblance to Sonja. Therefore, the resemblance was not only justified but vital.
While filming this movie, Michael Sheen was asked by Ron Howard if he could record a quick moment of dialogue for Frost/Nixon (2008). Sheen, covered in fake blood from the days filming, went to his trailer and recorded the dialogue. He asked someone to take a picture of him doing it and sent it to Ron Howard, who had it framed and displayed.
No mention is made of Lucian's father in the movie. His mother is shown as a werewolf, so there are two possibilities: (1) Lucian's father was another werewolf, and Lucian is simply a mutant, or (2) Lucian's mother was human but got bitten and turned into a werewolf while she was pregnant, making Lucian half-human and half-wolf (lycan). Most viewers favor the second possibility.
One can notice several instances of lycans/werewolves losing only the top half of their head in the film. This is due to the special effects crew re-using the cosmetics used for William in "Evolution", who was gruesomely slain in the same way.
The original Underworld (2003) only had two lycan costumes. Underworld: Evolution (2006) afforded to make three additional ones to portray the first generation of more wolf-like lycans as well as the character William. For this film the special effects team pooled the five available costumes, created new less advanced costumes for background crowd shots and re-used the werewolf costumes from Dog Soldiers (2002) to bump up the number to fourteen.