I've been reading this great trilogy lately. I don't want to name it, because I'm going to mention something that might be considered a bit "spoilery". The thing I'm loving is how the characters do something big, with much celebration and a sense of accomplishment, but soon thereafter they start to have the sneaking suspicion that maybe they shouldn't have done that. Even better, they realise they'd been missing obvious clues about the unintended consequences all along. It's big fun.
The series does the same thing with its characters, too. As the books go by, we learn more about them. There are sides to them we hadn't seen before. But right from the beginning the characters' behaviours have been shaped by the stuff we didn't know yet. What seemed like eccentric behaviour when we first met them turned out to be perfectly reasonable, driven by things we just hadn't known.
People are like that. When someone new comes into our lives, we don't know much about them. We learn about them bit by bit, and we may find out stuff about them that we wouldn't have imagined. Stuff that explains why they do what they do, or why they believe what they believe. Maybe knowing more about them makes you grow closer, or maybe it makes you grow apart.
I'm trying to keep this sort of stuff in mind with my own characters. I'm about 70000 words into the fourth (and last?) book of the Hybrid series, and I'm not even halfway done. I'm at the point where the characters are revealing stuff about themselves that the readers (and other characters) didn't already know. My goal is for the reader to learn something about the character and feel that it explains some of their past behaviour.
Because it's me, I'm also thinking ahead to what comes after the Hybrid series. There are two strong contenders jockeying for position in my head; I'll have to wait and see which of them escapes first.