Jemaine Clement and Taika Waititi wrote 150 pages of actual script but chose not to show it to a single person involved in the film (both crew and cast). The reasoning was to keep things spontaneous and actors to be surprised by the events unfolding before their eyes.
The man who plays Stu is not a actor but actually Stu Rutherford. A part-time business analyst for a Wellington company, LanWorx. He was hired for the film under the impression that he would be working on computers, and that he would play a small part in the film.
The hill, where the vampires have a run-in with the werewolves, is the same hill filmed in The Lord of the Rings: The Fellowship of the Ring (2001), where Frodo and the hobbits are running and hiding from the Black Rider under the roots of a tree.
The character of Petyr was conceived as devoid of personality, but unscripted scenes of the actor in makeup were expensive so extra footage of the actor in makeup was shot, such as the scene of him listening to music.
Viago and Deacon came to New Zealand from Germany, though Deacon is originally from Romania, along with Petyr. It is implied that Vladislav is also from Romania, as his character is named after after Vlad the Impaler.
One of the portraits of Viago shown in the film identifies him as a count. Some sources including Wikipedia list Viago's full name as Viago von Blitzenberg. In a promotional video Taika Waititi filmed in character as Viago for the New Zealand dating site FindSomeone, his name is given as Viago von Grossenhymen. In "The Trial" episode of the TV series "What We Do In the Shadows," Viago introduces himself as Viago von Dorna Schmarten Scheden Heimburg.
During the scene in the bar when the viewer first meets Jackie, the song "The Captain" by The Phoenix Foundation is playing in the background. The band is based in Wellington, NZ, where the movie is set and filmed.
The trivia items below may give away important plot points.
They lost a charred, polystyrene skeleton/body during the shoot, after throwing it in the harbor for a funeral scene (the scene didn't end up in the film), and not realizing it was drifting away. Taika Waititi commented on the situation saying, "It was quite realistic looking, so we had to put a press release out saying that if anyone finds this charred body washed up on a beach, don't be alarmed, it's just a prop".