Diagetic:

In the Buffy episode, a great example of diagetic sound is the audible howling wind that suddenly comes through the class room when the the TA, presumably, kisses Buffy on the table. The sound, which is technically off camera as you don’t see the physical affects of the wind, originates from within the story itself. The sound adds to the horror aesthetics because it is caused by a supernatural change in setting, and seems out of place in the context of the classroom setting. It peaks the curiosity of the viewer, which adds to the suspense. I’m having a hard time distinguishing this sound effect from diagetic and non-diagetic. It’s tough to tell if the wind is actually happening, or just used as a dramatic sound effect. I’m going guess it’s part of the story.

Non-Diagetic:

In the Buffy episode, a great example of non-diagetic sound is the sharp orchestral “screech” when the TA places his hand on Buffy’s shoulder in the hallway. The sound is an example of non-diagetic sound because it is acting as mood music that does not originate from within the story-line, but rather exists outside the present setting. The sound contributes to the horror aesthetic because it accentuates the paranoia and sudden change of the TA from a human to some demon-vampire creature.

Mise-en-scene:

My favorite example mise-en-scene in the Buffy episode is the framing of the young girl holding the box in the hallway. This type of scene is fairly popular within the horror genre (e.g. The Shining, Poltergeists, ect.) because weird little children are terrifying. The little girl is staged at the end of a long hallway, with ominous lighting coming from beneath and to the side of her, presumably with no natural source. It adds a ton to the horror aesthetic because, as I said before, weird children are scary. They are generally viewed as innocent, so when we see some kind of demon child, the swing from innocent to evil is compounded by our expectations, which in turn horrifying.