For me, a character starts feeling real when they stop behaving like an idea and start behaving like themselves. That sounds obvious, but it is usually the clearest dividing line. An idea can be interesting for a while. A role in the story can be useful for a while. A trope can even be compelling for a while. But none of those things are the same as a person. A character becomes real to me when they develop enough voice, want, contradiction, interiority, and emotional specifi