1) Make a list of 15 characters first, and keep it to yourself for the moment. (That way you're not leading the questions asked to fit the characters.)
2) Ask your flist to post questions in the comments.
"4, 6, 12 & 15 are starting a band together, what is their name and what kind of music do they play? Also, who's the lead singer, and what instruments do the others play?"
3) After your flist has asked enough questions, round them up and answer them using the 15 characters you selected beforehand, and then post them.