We found the fairs to be a good way to gather a lot of information in a short period of time. You can get more perspective from speaking to a live person than you can get from a website. I would not expect the admissions reps to ask interview type questions (except basics like what are your interests…). On your end I would come with a list of specific questions you want answered. Questions that will give you a sense of student life, types of students, academics and support, etc.
Personally, we found that the way the admissions reps presented themselves was a pretty accurate representation of the cultures at their schools. Some of those first impressions made us cross schools off the list right away. On the plus side, the fairs introduced us to schools we had never heard of before, and we ended up visiting some of those schools.