Take a previously written short piece and add internal dialogue (main character) responding to each action your supporting character takes. Put the internal dialogue in italics. This action will help you flesh out the personality of your main character. Choose a response type: examples - loves the supporting character, hates the supporting character, finds the supporting character boring, thinks the supporting character is hiding something. By choosing a response type, you will add depth to your character and interest in why he or she feels that way as well as cause the reader to begin to evaluate the actions of the supporting character: is he worth loving, hating, suspecting, and so forth?
When you have finished, add behavior cues, tone to dialogue, expressions that support what is being thought internally. Then remove all the internal dialogue and see if the additions created greater complexity to the piece. Note: You may find you want to keep some of the internal dialogue.