I was minding my own business, writing along, when a character I didn’t invite into my novel showed up. He just started talking and I knew that he had something interesting to say. Did I let him stay? YEP.
Here’s what I think about early drafts and sometimes even later drafts; if a strong character appears, I should hear him out, try to see how he might fit into the story, what he can add. I think these characters don’t really appear out of nowhere. If you’re connected to your manuscript and you’re in the story, they show up out of a need.
And sometimes they’re some of the most interesting characters you write.