tikatu | Odd thought on characters

I'm trying to work on chapter 60 of The White Winds, and I'm finding that Fermat is getting really cranky. I'm not sure if this is because the last few chapters have been mainly about Alan, and he's not happy with that or what. But it's coming out in the writing. And I don't know if this is a good thing or not. I can see how this can develop: Fermat can be unhappy that the two friends are growing apart. He's younger than Alan, not as mature, and things have been piling up emotionally for him. And as Alan has branched out with his friendships, so can Fermat.

Then there's Sable. I had specific plans for this OC, and now... she's pushing her way into the story, (not as a Mary Sue as she's really a minor character) and more ideas are coming along as to what I can do with her. How does this happen? How many other writers find their characters shaping the narrative like this? I find it happening more and more.