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: After Lieutenant Lennox drives away, Charley rushes over to Evil's house and begs for some vampire-related advice. He agrees to help (at a cost). As Evil is asking where the vampire will likely strike first, he runs both hands through his hair then drops them to his side. But in the following shot, Evil's hands are instantly back on top of his head again (widescreen only). (00:21:35)
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)
Continuity mistake: When Charley is about to be thrown from the bedroom window by Jerry, he accidentally grabs Amy's picture from his desk and throws it to a fence post below. But watch closely and you'll see: this picture is the very same one that fell to the floor in a struggle just a second before Jerry lifted up the window. (00:29:55)
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)
Continuity mistake: As Peter watches Evil's wolf form drag itself across the floor in pain, the balusters and broken wood fragments on the floor rearrange themselves. (01:20:55)
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: When Peter Vincent is fighting back Jerry in bat form he is scratched on the cheek drawing blood. When the bat flies away and for the rest of the movie there is no scratch on Peter's cheek. (01:34:20)
Continuity mistake: In the scene in the burger joint when Amy grabs the kid's BBQ sandwich and smashes it in Charlie's face for ignoring her. First the two tomato slices fall completely off his face, then one slice reappears on his right cheek, and in the final shot no more tomatoes.
Continuity mistake: When Charlie and Amy run into the nightclub to get away from Jerry she is wearing a blue purple plaid jacket to match her skirt and blouse. The jacket disappears when Jerry takes her to the dance floor, and she is not carrying it or wearing it when Jerry makes his escape from the nightclub, kidnapping Amy. Then in the scene when Amy wakes up in Jerry's house dressed in the white gown, she begins to focus and look at the portraits on the wall. Directly below and to the right of the first portrait are her clothes, folded neatly over the back of a chair - purple skirt, blue blouse and the matching plaid jacket, which has magically reappeared.
Continuity mistake: When Peter presses the cross to Evil's forehead, it's level. But, in the shot when he pulls it off, it's tilted to create the X burn.
Continuity mistake: When Charlie and Peter find the bitten wounded Amy lying unconscious on the floor in Jerry's bedroom, she is shaking as she transforms into a vampire. Her hair also changes back and forth between the shots. From a perfect curly coif to a longer frizzy vampish do. Then when Peter enters the room alone, and she gets up, it's suddenly long and curly down her shoulders.
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.