Signs your side character deserved the whole book!!
β You know exactly what they want and why they can't have it. The protagonist has no idea what they want.
β They make one offhand comment in chapter three and you're still thinking about it
β Their backstory is mentioned in two sentences and somehow richer than the main plot
β Every scene they're in is more interesting than every scene they're not in
β They have opinions. actual opinions. about things unrelated to the protagonist.
β You wrote them to be comic relief and they became the emotional core of the book. Congrats.
β Their relationship with the protagonist is more interesting than the main romance
β They die in act two and the book never recovers.
β Their one scene with the villain is better written than any scene with the hero
β Readers ask about them specifically. always them. you know the one.
β they have a life happening offscreen that you didn't plan but clearly exists
β Their flaw is specific and earned and costs them something real
β They disagree with the protagonist and they're right
β they want something from the story that the story never gives them (devastating)
β their friendship with the protagonist is more convincing than anything romantic in the book
β disappears for fifty pages and when they come back you notice immediately
β you know what their apartment looks like. you've never described it. you just know.
β they carry the theme of the book more clearly than the protagonist does
β the readers forgive them for things they would never forgive the main character for
β they make the right choice for the wrong reasons and it costs them everything and nobody notices but the reader
β you think about writing their book. you have thought about it for three years. you will write it eventually. probably.