I saw a question on Twitter about whether characters develop. I answered yes. My experience is not phenomenal. But I do know one thing.
You start with a situation, a guy in that situation and after that is not in your hands anymore. I know, Stephen King said that much earlier. But even if you plan your story in advance by snow flake, three tier or what ever method you like or keep all that in your mind as happens in real life, sometimes characters in a fiction book take on a life of their own. And if this happens it best to just keep on writing. The story will take you to its own end. You only write the epitaph.
Now I find stuff hidden in the deepest recesses of my mind express themselves from the characters, and also in this blog.
So I say, like I said in the post on Creativity: Just let it flow. You will struggle with the editing part. You may have to quarrel with the editor from your publisher. I did, and it is holding up my new Novel! But better that you suffer than your creation.
I would hate to call them my creations. They are not. They create themselves. Pop out of nowhere. They get into situations while you watch and tell them not to do that.