Often, the qualities of other people that annoy us the most are those that remind us of something within ourselves, a fault or lack of our own.
You might dislike a friend who brags about their accomplishments, when the real problem is that you don’t talk about your own and therefore feel overlooked. You may get angry at your spouse for having outsized influence over your joint decisions, when actually the problem is that you doubt your decision-making skills or are happy to cede your agency to another person.
You can use this human foible to define your characters.
Perhaps your protagonist is indecisive. You can show them struggling to make a decision but if you want to try something more subtle, put them in conflict with a character who has no trouble making decisions and acting on them. Your protagonist’s animus towards other person will underscore the quality they lack. You’ll lay the groundwork for your reader so they recognize what’s happening the next time your character struggles to make a decision.
That’s a very simple example of a complicated – and quite common – human trait. Consider how you might use this in your writing to define your characters’ personalities and create emotional conflict.