Get an explicit understanding of what is expected of you from the event organizers. After the crowd, they are the most important people to satisfy if you want

Get an explicit understanding of what is expected of you from the event organizers. After the crowd, they are the most important people to satisfy if you want to be on the future short list :) With that in mind, prior research on the crowd is great to have. If you can get a DVD, Google Video, or other archive of the conference prior, you can get a good baseline on what might be expected of a speaker and how the audience reacts. When you say "lead a discussion" if that means a panel or you acting as a moderator for others, it is good to know the personalities that will share stage. Definitely reach out to them for conference calls (if the organizer has not already) to map out what you plan to say and build quorum around time limits, pacing, and what you feel the crowd gets from the talk. If you have access to similar audiences or students of those profs prior to the event you could offer to provide pre-event talks and collect surveys to grade how you performed. Lastly, make sure you go easy on the food and drink prior to getting on stage ;-)