It started out tragic and now it's lightening up

Wolfe: Murder started out with the protagonist in misery, self-doubt, and pain. As he goes through the story, he's finding hope and some fun in life so his demeanor is changing from dour to lighter with some smiles and quips. Is this realistic? Maybe it's what's supposed to happen. I just have to keep putting words on the screen and see that happens. How to maintain some control?

Do you allow a character to take over a book?

Working on the second in the Wolfe series - a mystery - and I have a character based on a friend of mine. That character is emerging as a larger part of the story than I first intended. In fact, this 'person' may well take focus away from Wolfe. At this point (still on the first draft) I'm going to just let it happen. Then revise as necessary. Interesting what happens in the process, eh?