Many things about a character, a setting, and a relationship are unsaid. These are things that everyone understand are there, but either don’t see them any more because they are so normal or are things that they have learned to not say. Yet these unsaid things provide pressures and distortions to relationships. When unsaid things are missing, then relationships start seeming too pat.
For example, a Jim Crow era book may never talk about racism, yet you will see signs of that racism all the time. To have a Jim Crow era book without that racism would be strange. You would not get the correct tensions, even if the book was about something other than Jim Crow tensions. The unsaid will affect the way that the white homeowner speaks to his black gardener, and the way that the black gardener speaks to the white house owner.
A character has filters over their eyes. They all have things that they are blind to and things that stand out to them. These filters are part of the unsaid.
It’s not just racism that is unsaid. Two sisters may be mad at each other, and you see it in their dialog, but neither talks about why they are mad. Perhaps its mentioned somewhere, and the grievance is legitimate, but for most of the book, the grievance becomes the unsaid, an invisible force acting within and among the characters.
Sexual tension all lies in the unsaid. You see two people who are mad about one another, but they just don’t act on it. There is a gulf between them. The character never acknowledge this tension, because to touch that would acknowledge the tension and disarm it.
All you need to do as a writer is to determine a few things unsaid about a character. They don’t have to be important or even critical, but they do have to be things that push against a character.