In this fic Buffy, for a reason I can't remember, turns into something else. She's not a vamp but she's more than just a Slayer (small fangs, colored eyes, maybe). The scene I remember most is one where she goes into "game face" for the first time in front of Spike and he reacts very primally and instinctually to her (and she to him). I think he vamps and pushes her up against a wall and there may be some mutual sniffing. I remember it was really hot and they couldn't control their reactions to each other while changed. | 
