One of the benefits of doing the initial read-through is that you can see the contrast between your characters at the beginning and your characters at the end. When you finish a draft, you remember the characters as they are at the end. But reading through the first part of the novel can be a surprise. Wow, was my character really that whiny? When did they change goals like that? Oh my gosh, what happened to her new pizza store? Why is she running an auto repair shop?
The characters we have in our head before we write are static, but as soon as they touch the story, they become dynamic beings, acting on goals, reacting to events, learning and growing until, at the end of the story, they are changed for better or for worse. That’s part of what story is; change in your character.
What you need to identify in revision are those parts of the character that changed not because of the story, but because you found a greater truth in your character. Somewhere along the way you decided that Bob should be a florist, not an air-force pilot, because it’s more truthful to who he is.
And it’s quite easy to forget, when you write “The End”, that your character started the novel with the goal to work her way out of debt with a slightly dodgy mail-order business. But reading back, you realised (usually around 25-30k words in, I find) that it is more true to the character for her to find her missing sister and address the cause of her destitution, which is her unrelenting punishment of all friends and family, but ultimately herself.
So what do you do with a character who started as A, but really should have started as B? Usually it means a serious rewrite of the beginning of the book. You need to bring your knowledge of the character back and revisit his/her actions before the remodel. Look at her goals, her self, her reactions. Do they match the character you have written at the end of the book? Keep that person in mind as you revisit the events of the story.
Characters suddenly getting “interesting” after the first third of the novel is a fairly common comment in reviews, and it comes down to authors not aligning the person at the end of the book with the person at the beginning. Don’t make that mistake. Just as plots need to be tightened and trimmed, so do characters need to be fully realised from beginning to end.