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?' 'Write a drabble in which three and eleven fight crime.' (...possibly not technically a question.)
3) After your f'list has asked enough questions, round them up and answer them using the fifteen characters you selected beforehand, then post them.
Come on, play with me!