The flashback was of movie-Buffy being strangled/stabbed by hands reaching through her bedroom wall (without breaking the wall; the body was apparently desolid). Then her movie sidekick (did she have a sidekick in the movie? I know my brain made all of this up) staked the vampire behind the hands.
Scene returned to the present, with TV-Buffy and whoever her friends are in her bedroom, and Buffy is saying "Of course I cleaned the blood off the wall afterwards". The wall was now covered in posters and papers from her childhood: teen star posters and childhood drawings and whatnot pinned or taped to the wall, the way kids do.
A disembodied, vaguely threatening voice said, "You know you want to see what's underneath." Buffy & co. started taking down the top layer of posters and papers, and underneath were letters Buffy didn't remember writing to a person she didn't remember knowing. They were still trying to get to the beginning of the sequence when the dream moved on.
The other bit I remember is the crew of teens moving a bunch of corpses, and one of the characters tried to start a musical number (apparently because my mind remembers there's a musical episode of Buffy even though I've never seen it.) The character was trying to talk about the obligations the dead people still had, and started singing, in Fluttershy's voice, o/~"When you have a duty ... "o/~
And the other characters answered with "They're dead," in flat, not-singing, not-going-along with this voices.
Fluttershy-girl: o/~"You still have a duty!"o/~
"No they don't! They're dead!"
It didn't really go off.
The funny thing is that after I woke up I kept trying to make the song scan in my head, with the counterpoint sung. Even though the point in the dream had been that the counterpoint wasn't sung.
I can still hear Fluttershy's voice singing that line, which makes me wonder if I got it from an MLP episode and just forgot. In which case someone will surely let me know. :)