I'm really curious as to what other artists do at cons. To me, there's a strange kind of fun to work with ink in a book without penciling anything first and having that kind of "danger" of screwing up every time and still coming up with a drawing you haven't drawn before. It's as close to any kind of performance "improv" I'll ever get and I really enjoy it.
If you look closely, there are a few where I started doing 3 panel stories in the front of the book -- each different and written on the fly. I think I only did 3 or 4 of those -- the writing and drawing just took too long for people waiting to get their books so I went back to straight sketches.