This sometimes happens to me. The plot seems to be chugging along nicely, then I'll write something that seems to be what I originally planned - then look at it again and think "Wait a minute, no way does _____ do THAT! She'd want to _____" And once that decision is made the characters are starting to get out of control, and the decisions are being made depending on who the characters are, rather than the needs of the plot. Usually this ends up as a much better story - for example, in one of my Buffyverse stories someone killed Angel for excellent reasons, and until I realised that she'd do it, and why, it was going to be a much weaker story.
