Ok so I'm looking for an AU fic but I don't really remember it that well so this might be kinda vague. So in this fic everyone except Spike is pretty much exactually the same, Buffy is the slayer, Giles is her Watcher, etc. Except Spike who is a regular human with no knowledge of anything supernatural, (or maybe they just assume he doesn't I'm not sure, just remember he's definitely human.)
I think there's a part where Giles warns her not to tell him her secret idenitity, but she tells him to back off. Hope this is enough to go on.