1) Make a list of fifteen characters first, and keep it to yourself for the moment.
2) Ask your f-list to post questions in the comments. For example: "One, nine, and fifteen are chosen by a prophecy to save the world from four. Do they succeed?", "Under what circumstances might five and fourteen fall in love?", "Which character on the list would you most want on your side in a zombie invasion?"
3) After your f-list has stopped asking questions, round them up and answer them using the fifteen characters you selected beforehand, then post them.