So I love writing letters. I have a pen pal and who's really awesome. Anyways, here's an exercise.
Pretend that you're pen pals with your characters and write them a letter. Then pretend to have them write you back. How would they respond to what you said? What kinds of things would they write about?
Then, have your characters write each other letters! Your MC could write to your villain and demand to know why they're such a villain or they could write to a minor character and lament about why everything always seems to happen to them.
Have fun and be creative!