How do the characters react to one another in the story? One rule to follow in both fiction and playwriting is that for every action, there should also be a reaction. How does the way in which the character speaks to others impact them? What are important scenes where a character does something that has a direct impact on another character that may deeply affect the story?
You are also developing the personality of your characters, specifically your main characters. How do other characters behave when they are around the protagonist or the antagonist?