Arguing over themes
At school, many of us were taught about themes as big abstract concepts, a Worthy Thing lumped onto the narrative. In writing, themes are the topics we care passionately about, that make us see red or cheer, that make us leap into an argument or withdraw from a conversation: they come from our deeply held beliefs and convictions. At their best, they're morally complex and give the reader something to chew about. They're our character motivation in the story, as writers.
Pick a topic you feel strongly about. If you've been using the Twelve Days of Writing already, look through what you've done so far, to see if any issues you care about are already emerging. Flip through a magazine and pull out two pictures of people you like the look of. (If you did the Characters prompt, you can pick two of those characters.) Give them opposing views on the topic: try to give each one the opposite view to what you'd expect them to hold.
Write a discussion or argument between them, as heated or calm as you like. Have them refer to their own experiences and lives, to make their points (as you make up extra details willy-nilly). Let the argument range widely, all around the topic, and make the view you don't share as convincing as the one you do. (Note: for this exercise, you can just write pure dialogue without action - if you want to add action, that's up to you.)
Tomorrow we're writing with... Subplots.