One challenge of writing a long multi-chapter story, is that we ourselves, the authors, change over time. For those of us, like myself, who take at least a year to write a book, the changes can become very pronounced. So, not only do we have to keep our facts untangled, but we have to speak in a voice that remains in agreement with itself. For instance, if we spent part of the writing period process depressed, and part happy-go-lucky, it can be difficult to maintain the personality of our characters evenly throughout the story.
It greatly helps to have a protagonist who is also undergoing evolution during the portrayed period of their life, such as Timmy. Other stories, those where a steadfast hero is needed, are not nearly as easy to keep believably consistent.
I have illustrated a book. There were 14 pictures, and they also took about a year to complete. I found the same problem happening - the nature of my artwork wanted to change over time, and yet it was important that all of the illustrations reveal the same person. There are tricks that work in illustration, such as creating the pictures out of order so that the changes are not so apparent. Writing a book takes a different approach.
I have to close my eyes before I start writing each day, and imagine all my characters. Then, I let them walk about in my head, and do what they are going to do without input from me. Funny, they always know what they're doing, they've always got a direction, and once I get out of their way, they go right down that road. If I get stuck while writing, I just shut my eyes, and let go again, and off they scoot. Usually, at that point, it's all I can do to type fast enough to keep up with them.
I know other people write very differently from me. They are in control of their story, and I greatly respect that way of doing things. I am not so much in control. Er, I don't think so while I'm doing it. Yet, in the end, I seem to have written myself to exactly where I need to go. I am very grateful that a couple of people around here like to read what my characters take upon themselves to do. :)