Recently I had a slight, incidental character suddenly morph into a substantial, main character.
I know other writers have had this experience of the Character That Would Not Stay Put. You place him in a marginal role—he’s just there as reference, a mere conversation piece—and he shouts ‘NO!’ and starts showing up in person, gaining definition, taking over the place—now he’s courting the heroine—what cheek!
I think this is one of the most magical aspects of writing. The surprises from within the self. The spark of imagination that causes a seismic shift, tilts your writing world sideways…
You might want to run around the room screaming “how could you do this to me?” but in the end you go with it because you know its the right thing to do.