The 'attack dogs' outside of the vampire mansion were actually very docile and playful canines, which were the only dogs available at the time. For the scene where they chase Scott Speedman, director Len Wiseman had to film short clips of the dogs running and later put in sounds of vicious barking. If you look closely though you can see their tails happily wagging back and forth.
A kiss between Michael and Selene (when she unchains him in Lucian's lab) appears in the trailer but was not included in the theatrical release or on the DVD, but it is on the unrated version of Underworld.
Not only in this movie did Kate Beckingsale meet her future husband Director Len Wismen, but at the time she was in a long term relationship with actor Michael Sheen, who played Lucian. Michael and Kate share a daughter together who appears in the film during a flashback of when Selene was younger. Michael and Len have since become very close friends and have collaborated with each other on Rise of the Lycans. Michael and Kate still remain very close and both are raising their daughter equally. Kate has stated that Micheal is her best friend and that they still remain very close.
Len Wiseman opted to shoot many of the action and effects sequences live, without computer imagery. In one scene, for instance, a werewolf seems to run more than 50 mph behind a speeding car. This was shot using an elaborate rig towed behind a vehicle, with actor Michael Sheen doing the closeups of the running and the jump onto the car, while the long shot of Lucian running was done by stuntman Todd Schneider.
The name of the city where the film took place is never mentioned. However Michael's address that we can see on Selene's computer is an apparently misspelled Hungarian address (it should be "Lakatos József u. 39." while it is spelled as "Laktos Joszef 39 ut." instead). You can also read "Eötvös utca", a street in the historical downtown of Budapest, many times while driving by to the secret interrogation place and there is an ad in the subway station where the shooting takes place in the beginning of the movie referring to Szentendre which is a small town near Budapest (obviously left there as non-Hungarian viewers don't understand the text anyway).
The ancient tome which Selene uses to research the history of Kraven features pages from three different sources, all shown before "The Fall of Lucian." The first, featured on the page with the seals of Amelia, Viktor, and Marcus, is an ancient Hungarian text entitled Funeral Sermon and Prayer. The second is an old Catholic Roman Missal, including the Propers of the Second Mass for All Souls' Day. The third is a book entitled Instructissima Bibliotheca Manualis Concionatoria, which is also featured between pages of the story of the fall of Lucian.
The underground tunnels used for the Lycan/vampire battle at the end of the movie were mostly shots of the same small rubble set, but shot from different angles and lighting in order to make it look like different locations underground. During the DVD commentary, director Len Wiseman laughed at this fact and pointed out the scenes in which this trick photography was used.
White Wolf, Inc. and author Nancy A. Collins filed suit against Sony Pictures, Screen Gems, and Lakeshore Entertainment on 4 September 2003 for copyright infringement little more than a week before the theatrical release of Underworld, alleging 17 counts of copyright infringement, and claiming over seventy points of unique similarity between White Wolf's role-playing games, "Vampire: The Masquerade", "Werewolf: The Apocalypse", and their creation, "The World of Darkness", in which the games are set. Nancy A. Collins is the author of a short story, "The Love of Monsters", published in 1994 by White Wolf Inc, and set in the World of Darkness, which she claims the entire plot of Underworld is based on. The suit was settled out of court, for an undisclosed amount.
Although many weapons are used in this film, the iconic handguns in the subway scene are the Beretta 92FS, heavily modified to look like the Beretta 93R. They have also been modified to fire in full automatic mode (3-Shot Burst, actually; If you slow down the scenes you can see 3 rounds fire per trigger pull) and contain barrel weights. They use 20 or 30 round box magazines.
When Selene first encounters Lucien, she gets cornered by Lycans and fires her guns through the floor in order to escape. Eleven years prior, this cinematic stunt was performed by the main character in the 1992 film "Nemesis."
In the newly released 2017 4k remaster of Underworld, the scene where Michael recounts the events of Sonya's demise when he speaks with Lucian in the film is replaced with the actual scene from Underworld: Rise of the Lycans instead of the original flashbacks used in the 2003 version.
Michael sheen plays a Lycan (aka) a werewolf who dislikes vampires because of the war in which his vampire girlfriend was killed in a later movie however he plays a vampire who hates werewolves in the Twilight Saga.
In the 2017 4K High Definition remaster of Underworld, the scene where Michael sees flashbacks into Lucian's past/Sonya's death was actually replaced with the 2009 scenes in Rise of the Lycans instead of the 2003 flashbacks.
"Underworld" was lightly parodied as "The Wreathing" in the 2010 BBC radio/audiobook adaptation of the Simon Brett's "Cast, in Order of Disappearance". The original story (written in 1975) was relocated to the set of the vampire film for the audiobook, and starred Bill Nighy ("Viktor" in the Underworld films) as Charles Paris, a minor British actor and amateur sleuth, engaged to play the role of "Szabec", a middle management vampire in an organised vampire society. The mystery is set around the actors while on and off set for the film. Martine McCutcheon plays a character which is loosely modelled on Kate Beckinsale.