Other mistake: When Amy and Evil are at Peter Vincent's house to convince him to help Charlie, you see them looking into a horizontal mirror that's on the wall on the left side of the screen. If you actually look at the reflection, Evil is on Amy's right. However, he's on her left in reality.
Continuity mistake: When Evil is below the staircase transforming from wolf into man, he grabs hold of the table leg lodged in his chest. Between shots he goes from using one hand, then two. (01:21:50)
Continuity mistake: Right before Jerry busts through a stained glass window, Peter can be seen holding a wooden table leg - which was driven through Evil's wolf form earlier. In the following shot the leg is nowhere to be seen. (01:32:00)
Continuity mistake: After Amy punches Evil and shoves him aside, he grabs hold of his right arm. Between shots, he lets go of his arm. (00:41:35)
Revealing mistake: As Jerry leaves Mrs. Brewster's bedroom, a mirror reflection shows the door opening a crack before he even touches the knob. (00:27:00)
Revealing mistake: During the basement finale, lines can be seen suspending Jerry in the air when he lunges towards Peter and sends him crashing into some shelves stacked with glass bottles. (01:37:55)
Revealing mistake: When Jerry is dragging Charley across the bedroom by his neck, he turns around and slams him into a wall. It wouldn't be unreasonable to assume that once his head makes contact, the wall's gonna break somewhere. But notice how the wall is already cracked right above the classic convertible picture a split second before it should be (widescreen only). (00:29:05)
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.