Are you aware how your facial expressions or body language signal what you’re feeling? My face often says what I’m thinking, to my detriment.
Consider how your characters can convey their inner thoughts, without you having to state them expressly. Do their eyes crinkle when they’re trying not to show anger? Do they frown or shuffle their feet when they are afraid of disappointing someone? Does their weight shift from one leg to the other when they are uncomfortable? Do they tug at their collar when they lie?
This also works in reverse. Other characters will send your protagonist signals, whether they realize it or not. You don’t have to turn your hero into a human lie detector to work with body language, though that could be fun, too.
As you go about your day, pay attention to the signals other people send you during your encounters.