There's an interesting thread in Dark Forest by Cixin Liu (second book of the Three Body Problem trilogy (yes, I know I'm being boring constantly recommending them here but these are
really good books)). I actually wonder whether it's a real thing.
A short (minor) spoiler follows.
One of the protagonists is married to a writer. He follows her advice about writing, and in the process of developing a character he manages to, well, instantiate a pretty much living version of that character in his own mind - a version that grows and thinks independently of its creator. He eventually gets emotionally involved with his imaginary creation, and later learns that it's something that happens to good writers, and that his wife has such a character "living" with her too.
What I wonder is whether it's really the case that good writers actually develop their characters to the point of them turning pretty much into tulpas? A kind of background thread in their brains, with separate micro-consciousness.