Feels like Christmas morning!
As established early on; a secret from the protagonists but shown to us, a serial killer is hiding in the attic of the sorority house they belong to. We're always a step ahead of the victims who at first believe they are dealing with nothing more than a prank calling creep. We see bodies piling up but they don't. As a plot device to add depth, it holds the grasping suspense for a very long time.
The human relationships are caricatural but authentic. Because Black Christmas is a slow burning horror thriller, this becomes critical in caring for bored women with little in common about to spend an increasingly awkward Christmas together. The girls have subtleties to their archetype and it gives us a little meat to chew on. This is a holiday movie, after all, and they provide necessary warmth.
The technique is sometimes flawed, the editing gets choppy, but the photography is just right for this kind of ambiance. Sound manipulation nicely comes into play in regards to the prank calls, giving the bad guy an oppressive presence, a spine-chilling monolog and pathologic verbal mannerism. Black Christmas makes us fear through apprehension, but it gets darker and ultimately visceral.