I'm still working on revisions here, big shocker. My biggest concern right now is tweaking some of the mystery elements to make the ending more fun for the reader, and a large part of that has been changing my characters. Not swapping out Bob* for Joe, but turning Bob into a different person than I first thought he would be. Sometimes I need to do this to alter the reader's perception of him, and sometimes because I realize that it just makes the story work better.
This is something I find pretty difficult. For the past year or more, Bob has been living in my head as an intense, commanding, moral person, and suddenly I need him to be friendlier, more down-to-earth, more sly. It takes me a while to get used to the new Bob. I like it though--especially when a character change makes a whole other row of other logical changes snap into place.
Do find that you need to change the personae of your characters during revisions? Or do you generally stick with them as you originally envisioned them, and change other things?
*Not the real character names, I'm still too chicken for that. Also don't want to give anything away. Just, ya know, in case... ;)