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