Continuity mistake: In the beginning of the film where Charlie watches Dandridge and Cole move the coffin into their house, there is a close-up shot of Amy removing her blouse. The film then cuts to a close-up again of Charlie and in the background Amy's blouse is still on (probably only visible in the widescreen version of the film). (00:05:20)
Continuity mistake: When Jerry has Charley pinned to the cracked bedroom wall by his neck, he suddenly drags him over to the window a few feet away - which is nailed securely to the frame - and lifts it up effortlessly. Next, we see Charley looking over to his right side, but now he's back up against the cracked wall in this shot, not a window. (00:29:50)
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.