To continue on this topic…
If you’re applying for a job with a specific employer (whose name is known to you; many times when you respond to an ad the ultimate employer is kept unnamed for various commercial reasons), then you should do some homework on that employer so you know a bit about their history, products, needs and plans (to the extent that those are public). Then you can ask questions about the things that interest you and others that not only show off what you know about the company (don’t do it just to be a show-off), but show that you’re thinking about their future expansion, future products and needs, etc., and how your interests dovetail with those expectations.
On the other hand, when it’s a more wide-open jobs fair, you can’t be expected to know a great deal about all of the potential employers there (unless, perhaps, they’ve been listed in advance and left some biographical / company information that you could have read up on). Even so, the nature of the event (though I’ve never been to one) is more like speed dating (which I’ve also never done – so don’t come to me as an expert in either field!). You need to find out as much as you can about each other in a short meeting to see if there’s a mutual interest worth pursuing.
I doubt if many people get hired “on the spot” at a jobs fair. But employers get to see a lot of “generally qualified” people who they might want to find out more about later, and potential employees get to meet the front office HR people at places that they may have only known (or not even known at all) as names on a roster of attendees. You should be looking at this event as “who would I like to ‘date’ again in this corporate way?”
So ask questions – DO ask questions – about the things that you’re interested in, to see if this potential employer is also interested in those things, too.