Ask them all to think of a moment in their lives that they're not too proud of. If someone could look back at this moment (perhaps a time they cheated on a test or let down a friend who was counting on them), what might this person think about them or their actions? This should help students think about issues of audience and the need to explain things fully to readers who have not lived their lives with them (something they sometimes forget). Then ask them to explain why they did what they did. This should emphasize the need for description, detail, and development.
When they have finished, ask them to consider what they think this moment might reveal about them. Then have them share their story with their group or a partner, and have the group or partner decide what they feel the episode reveals about the writer. Hopefully, these two purposes will match, and may give them an idea for an essay topic.