Crossposting to this thread, because this most certainly does qualify as a horror film.
Just watched Martha Marcy May Marlene.
Olsen was fantastic, I can't wait to see what she does in the future. John Hawkes was, well he's John Hawkes. The man can do no wrong.
The tension throughout the movie was insane. The sound (or lack thereof, I should say) really let the viewer feel Martha's isolation and that sense of being trapped in one's own head, as did Durkin's decision to use wide shots so often. That distant shot near the end of Martha floating in the water really stuck with me.
Hell of a debut from Sean Durkin.