Miss Allerdyce from "Burnt Offerings".

This movie frightened the shit out of me when I was growing up. And it was this scene in particular that still spooks me. The movie plot is confusing (look it up on IMDB) but I'll tell you this - the house this family is temporarily staying in "rebuilds" itself each summer by feeding on the lives of the family/caretakers. In this clip (which is near the end), Oliver Reed finds out that his wife has "transformed" into the old lady he thought his wife was looking after. Was there ever an old lady? It's confusing...just watch it.

(You're going to want to mute the jukebox at the very top of the blog before you watch any of these videos.) And no, Oliver Reed is not overacting at the end. His horrific screams are justified. Rent the movie. And scream yourself.