If I'm hosting a topic conference I read up on the topic in books, magazines or the web. I usually ask basic questions or set up scenarios which get the users thinking about the topic which helps lead and focus the discussion. Preparation of the canned material usually takes 1 to 2 hours. Other guests the audience love are Don Crabb, Chris Breen, Andy Ihnatko, and Tom Geller. They all don't mince words and really get to the heart of user questions.
Writers are great guests because they can be very creative in live chat.
Other company personnel are usually slightly timid at first, and type slower. They're more cautious because the medium isn't as comfortable for them. I don't think I've ever really had a bad guest. I don't look for a thing. I just try and find people who I find interesting, have a cool product to tell us about, or are engaged in some interesting activities.