What I remember:
Spike and Angel go to see Xander.
Xander is doing a seance-thing where he speaks to the dead and give their messages to the living (his audience).
Xander sees dead people ;)
Xander feels Angel and Spike is there because all their (dead) victims make him freak out.
Xander needs to help Spike and Angel with something and Spike stays with Xander.
They develope a relationship.
I keep thinking that the title of the fic is something like circus or freak but I'm not sure. I tried to google for it but my google-fu is not strong with me tonight :(
Found by sparrow2000 :)
It's Manifestation by Lazuli.