When we first create a character we may not be sure of his, well, character. We introduce him in a scene and decide we're fairly happy with him so he remains in the book. By book's end, we know him so thoroughly that we don't have to wonder what he'd do in a particular situation. We can predict his actions because he's fully formed.
So when we finally read our completed novel, how does this character sound in his early scenes? This is an important question and here's my advice. Be prepared to rewrite your character's early appearances to match your now-seasoned understanding of his makeup. There's a good chance he'll need at least minor touch-ups to make him comport with the personality you've come to know.
It's easy to see this problem in terms of some of your favorite TV shows. Think of a show you love and then consider the first episode of the series. Ask yourself this question: in the premiere episode, were the characters right? In that first episode were they the same characters who appeared in the third or seventh year of the show? For me, and probably for you, it is glaringly obvious when they're not.
Many good writers nail their characters down before they create their shows. Others fine-tune them along the way -- and this happens even in good shows. How many different colors was Data's skin in Star Trek? And how much did Worf's appearance and personality change during the series? Sometimes it's not nailed down beforehand but as fans we forgive the creators of the series. In fact, Data's changing colors are kind of fun to observe (as is the development of his actor's wrinkles).
The best sitcoms don't seem to have this problem. A few do, but not many of the big success stories. The characters are finely honed in the launch episode and they still act the same way five seasons down the road. That's good work on the part of the creators. (Mind you, characters can grow and change; that's not what I'm talking about here.)
You can usually get away with problems of this sort on TV because each episode is a standalone presentation -- but it's not that way with a book. Each chapter is not a new event that will be viewed in isolation. Readers remember. In fact, many readers will devour a whole book in one sitting and they, above all others, will certainly be aware of inconsistencies. There is only one book so we must keep our characters in line.
I hate to rewrite scenes as much as the next guy but it's worth it when you're refashioning a character to make him consistent. Don't hesitate to touch your characters up once you realize who they really are. It's always worth the effort.
Postscript: Scrivener can be very helpful in this regard because you can call up only those scenes in which a particular character appears, allowing you to see all of his or her scenes in one fell swoop no matter where they appear in the book. This is extremely helpful when trying to guarantee consistency in your characters.