Trivia: This can only be seen for a brief second on the widescreen side of the DVD. In the scene where Dandridge follows Charlie and Amy in Club Radio, there is a man standing by a mirrored column, and Dandridge walks behind him, and for a second you can see Dandridge's shoulder reflected in the mirror. Vampires do not have reflections.
Trivia: The car that Charlie drives belongs to director Tom Holland.
Trivia: The drink that Jerry has in his hand while sitting in the living room is a Bloody Mary.
Trivia: The character's name Peter Vincent is a combination of the first names of real-life actors Peter Cushing and Vincent Price, most known for their roles in vampire and other horror films.
Chosen answer: I would assume a "thrall" basically a ghoul that is attached to the vampire. A ghoul in old lore was created when a vampire gave blood to a living person, but did not take their blood first. They were then their daytime slaves, doing errands, finding victims for them, and took care of the vampire's body when the sun was out.
Billy was to Jerry... what Renfield was to Dracula. Slave, daytime watcher.