If someone in the story seems to act more like a stereotype than a person, you don’t need to accept that. Rely on curiosity rather than contradiction. If this person seems two-dimensional, dig into that mystery. What really motivates this person to act this way? Explore the question further, until you can understand and even sympathize with this person, even if her actions put her in direct conflict with your character.
This principle relates to our agenda to reveal everyone’s personhood by reminding us to dig deeper any time someone seems to lack personhood.