I think it's probably a tie between introspection and body language.
With introspection I just really love showing a character's train of thought as well as using it to sometimes be an unreliable narrator. I think it's really fun, especially with characters who tend to have low self esteem or are a bit pessimistic.
Body language I enjoy as someone who's used to watching it pretty closely (I've run in some interesting circles LOL) as I like showing tiny shifts in emotion or mood that a person may not even notice. Sometimes body language tells stories that words and feelings don't ♥