Sounds awesome, is there a youtube or screen cap of that?
In the original Dracula, there’s a scene where one of the vampirified (but still in the closet) women is smiling at an unsuspecting man who is talking to her. He looks away from her, still talking, and her focus shifts to his neck. The smile stays on her lips, but now it’s an obsessed “yummy, get a load of that pulsing artery” kind of smile. It’s right on the cusp between funny and major creepout.
A couple of excellent horror shorts if you really want to fuel your phobia for weird smiles:
What, Technically a Smile, The Unsmile, and Slasher Smile weren’t good enough for ya?
Let’s put a smile on that face… 
BTW I meant to add, the scene from Buffy mentioned above was definitely on my mind when I posted this. It was probably one of the few true scares on that show.
