Great question...I had been interested in this at times too and would love to hear of any first-hand experiences.
I think it's probably a little bit of both in terms of the Imagineer telling what he does as well as a Q and A.
Since it is done as a small group, I'd worry that the other diners could have totally different areas of interest as far as what is asked and discussed. I personally would want to know specifics in terms of the park design and the technology and what all happens in the process of putting attractions together. I'm also very interested in music for the attractions and would love to know the creative and recording process of putting all of that together.
How much the particular Imagineer at the dinner is involved in the area I want to know about, or how much others at the table would be interested in the same things as me would decide just how much of what I'd like to know about is discussed or talked about. This would be my greatest concern.