Character is a big topic for me.
When I write a story, it's all about the character (the area where I need help is usually plot). I tend to be obsessive about editing in chronological order because I want the story to reflect subtle changes in the character's judgment based on the progression of his or her experience.
I'm currently in the middle of a complete novel revision as a result of this operating principle, and I thought I could elaborate on the idea a little so you can see what I mean. I'll be trying to avoid spoilers, yet be clear.
The character: Dana. Some of you who have explored the site may recognize her name. She's a recent high school graduate heading into a grand adventure as she goes to college for the first time.
The revision: Dana was originally written as an only child, but now I'm giving her an older sister who went off to college the year before, failed, and is depressed and living at home.
The changes: These come in three types.
1. Factual changes. I'm reading through to make sure I don't ever forget that she now has a sister. This one requires attention so I make sure I don't miss any of the instances where a sister-reference might occur.
2. Character Judgment changes. This one is actually requires more attention than the first. The sister is not just a body; she brings with her an entire backstory and set of experiences. The trick is to get these in without actually writing out the backstory. I look for things like this:
Dana makes a comment about how hard it is to sleep in the same room with her new roommate; now she thinks of it in terms of "I haven't shared a room with Caitlyn since I was six."
Dana gets disturbed by the sound of crying in the dormitory hall; now she thinks of it in terms of the awful feeling she gets walking past Caitlyn's door at home.
Dana decides not to tell her parents that she wants to change her name; now she approaches Caitlyn first and her sister calls her stupid before she can even get the whole announcement out, so she never tells anyone else.
Dana hates to hear her mother order Caitlyn around; later when she hears other people getting ordered around, she reacts with extreme revulsion toward the person giving the orders.
3. Changes that make themselves. The best thing about this revision is the stuff I don't even have to change. Writing a story to me is like making a bell: adding a bit of material here or there can change the resonance of the whole, even in areas where not a single word has changed. In the case of story revisions, slight changes in the beginning of the story can drastically alter the feeling of drive in the story, and the sense of emotional magnitude associated with later successes and failures on the part of the protagonist.
The last thing I would say here is not to make changes for no good reason. In my case, I was looking for an opportunity to enhance drive and character motivation, and the "sister change" turned out to be the best option. But before I dived into rewriting the whole darned thing (yikes, but it's a chore!), I made sure to think through in my head some of the major repercussions of the change. When I realized they all looked good, good, good, that was when I took the plunge. Now I'm more excited about the book than ever.