Trying to figure out what this movie title is. . . .

I remember this guy (A) figures out that he is somehow turning into a zombie. Then he tells his guy friend (B) and this friend becomes a zombie but then is decapitated somehow. So friend (B’s) head is sitting on a shelf and he is trying to say something but since he has no body he can only move his mouth. So friend (A) presses a vibrator up to his throat to hear what he is asking.