1. A term sometimes used by writers to describe the common phenomenon in which they feel one or more of their characters has taken on a "life of its own" and talks back to them or pushes the plot in their own direction.
2. A strong attachment to a fictional or historical character and/or the story of said character; a habit of holding mental "conversations" with such a character; a sort of "imaginary friend" for grown-ups.
The story ended up going in a completely different direction than intended, as the author accidentally soulbonded the lead character, who then refused to be killed off.