One of the most important considerations, and one I see forgotten frequently, is that the person giving the testimonial be mic-ed, with the input going directly into the camera. I see a lot of demo reels where it is difficult to hear the testimonial -- or it just sounds amateurish because the testimonial is at the same level as the ambient noise (and there is a lot of ambient noise following an event).
Definitely have questions prepped. Even the most enthusiastic evangelists for the show freeze up in front of the camera; they're self-conscious, it feels artificial, and they don't know where to start...
The first time I brought a videographer in for my show, I hired a news cameraman. While the footage of the show suffered (he was used to working with a director and shooting inserts that would then be montaged together in a news story -- and I did a poor job of communicating to him that what I needed was a product that could either be watched start to finish, or montaged together...so he was constantly zooming in and out and it was impossible to watch any single routine straight through), he was awesome at getting testimonials (and did so without me asking)...he worked the room after the show interviewing the guests (what did it add to the event? what was your favorite part?) and that's the model I have since followed
When crafting the questions, especially for the client, do every thing you can to bring out the benefits of working with you. Why did they hire you? What results did they get? This is an opportunity not just to get killer footage, but to try to crawl inside their brain and understand what they want for the next time you write copy.
Good luck with the shoot! And don't sweat it, it is a learning process and your reel will constantly grow. Just keep taking every opportunity to get tape...