Board Thread:General Discussion/@comment-25548156-20140918215148/@comment-8161155-20141023015043

I think that everyone saw Bill as Dipper and heard Dipper's voice when he spoke; Dipper could just see Bill as he really was because he wasn't *really* there. Also, it would explain why no one cared that Dipper was apparently talking to thin air; only Dipper could see that.