We do this fairly frequently an it is always a highlight of a trip, but please be aware that the discussions tend to be more about theming, storyline, guest experience, scenery/set design, park atmosphere than about the actual physics and technical stuff.
It is a multi-course lunch from a set menu, and your imagineer is there for the entire meal. There are eight spaces available at each lunch, and unless your party is eight then you will be sharing the table with others. We have had anything from four to eight total people at lunch with us.
There is plenty of time for conversation and photographs, and you will each receive a commemorative glass plate which the imagineer will autograph for you. We usually just ask the man at the podium in the brown derby to hold it for us so we don't break it in the park during the afternoon.