Toby Whithouse originally conceived the show as a drama in the vein of This Life (1996) with the three characters sharing a flat. Originally, Mitchell was a sex addict, George had anger-management problems and Annie was an agoraphobe. Whithouse added the supernatural elements later, finding what he believed to be the underworld equivalent for each character: the sex addict Mitchell became a vampire, the angry man George became a werewolf, and the agoraphobe Annie became a ghost.
The producers of the show felt that it would stretch the budget to the limit if they had to digitally remove Aidan Turner's reflection from everything that had a reflective surface, so they came up with the notion that vampires can only be invisible in objects with silver backings, such as mirrors and cameras.
According to Aidan Turner in a BBC3 question-and-answer session, Mitchell's fingerless gloves were more necessity than fashion statement due to the freezing set temperatures filming in wintertime Bristol.
Mark Gatiss portrays the Vampire overlord, Mr. Snow, in Series 4. The name is given to him for his icy demeanor. Ironically, Gatiss's character on "Sherlock", Mycroft, was nicknamed "the Ice Man" by series baddie, Jim Moriarty. The two characters even share a similar wardrobe.
The trivia item below may give away important plot points.
Over the course of the show, none of the main characters appears in every episode. The longest serving character was Lenora Crichlow as Annie, appearing in 30 of the series' 37 episodes, and the shortest being Kate Bracken as Alex in 9 episodes.