When it comes to learning about your characters, we can get information from a variety of sources.
- Descriptions you provide as the author.
- Things the character says or does
- Things other characters say and do around the character
- And introspection
The couple sat quietly across the dinner table. Sally quietly asked Dave, "Is anything bugging you?"
"No, it was a tough day at work, that's all" Dave said squirming in his chair.
Now, with these two lines, we can sense something is not right at the dinner table. The adjectives the author used give us that feeling, but other than the empty words, we really don't know anything. It is here where the introspection would come into play.
If this seen was in Sally's POV we want to know the thoughts running through her head. What was it she was seeing in Dave that would make her break this silence. If it was quite for a long time, we would want to hear that inner dialogue going on in her head to build the tension. It would also give us more of a sense of her concern and worry.
If this was in Dave's head, we would get the same thing, but we would also (my bet) know more about why he was squirming in his seat. Did something happen during the day that would have set up this uncomfortable situation. How does he feel about it? Is this messing with his internal conflict and now possibly creating an external conflict between the two of them.
I do know that a lot of new authors are told to hold back from huge blocks of narration. This does not mean to eliminate it all together. It is fine to include internal conversation and narrative. It is here where we really get to the heart of the character.