I've been on both sides of the table at an acccounting career fair..once as a senior in college looking for a job, and again the next year representing my firm.
My experience was in looking and recruiting for an audit position:
Student: Its crowded, each firm only has a few representatives, and you are competing with hundreds of other students for face time. I remember waiting in line forever behind other students for a chance to talk to the rep (partner, associate, etc.). It can be VERY difficult to get in any kind of personal face time when it's crowded and loud,and often other students will try to get in on the conversation and squeeze in their face time too. Get there too early, and the firms dont have anyone to compare you too, so you wont stand out as much. Get there too late, and they think you werent that interested in attending, or they run out of business cards and you wont have anything to follow up on.
Firm: Biggest secret I didn't know as a student: behind the table there are two stacks, a MAYBE and a HELL NO stack. We took every resume we were given, and put it one of those two stacks. Some of us made notes on the resumes too, especially if we really liked the person. I was shocked at how many students came up to me and admitted that they were desperate to find a job, they didn't know anything about my firm, and frankly they didnt care to ask, they just wanted to know if we were hiring or not. Those resumes went in the HELL NO stack. Then there were the students who didn't know anything about auditing or public accounting. In my opinion, dont ask “so what does an auditor do” or “what is your job like?” because if you were serious about auditing as a career, then you would have done some basic research online and already know what the basic job duties are. Something more like “so how many engagements do first year staff typically manage at one time?” “How much does a first year auditor get to actually work on the report?”. To me, those are better questions. Or questions about industry specialization are good too, because you probably want to know the kinds of clients you would be working on, and then you can bring up something smart about that industry and impress the person you're talking to.
Be personable and likeable. You want them to think”ya, i could be stuck in a tiny conference room for a week with that guy”. Auditing is actually a very social and team oriented career, so you really want to fit in with the firm.