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