As someone once advised, “Grow antennae, not horns.” If you prejudge someone as shallow or crazy or ill-informed, you automatically cease paying attention to what they say.
So a basic rule of listening is to judge only after you’ve heard and evaluated what they say. Don’t jump to conclusions based on how they look, or what you’ve heard about them, or whether they’re nervous.
In fact, maybe a good exercise would be to go out of your way to listen to a difficult speaker. Maybe he talks with a thick accent. Or talks much more rapidly, or more slowly, than you, or uses a lot of big words.