1) Make a list of fifteen characters first, and keep it to yourself for the moment. That way you're not fitting the characters to the questions.
2) Ask your flist 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 seven fall in love?'
'which character on the list would you most want on your side in a zombie invasion?')
3) After your flist has asked enough questions, round them up and answer them using the fifteen characters you selected beforehand, then post them..