Today I did my first usability test and I can see why it takes a practiced hand to get good at it. It is basically the opposite of a normal social situation. One person uses a system and does all the talking and you don't do anything that might change the way the person is thinking as they go through the tasks. You have to really try hard to to react or respond.
The hardest part is when people are way off with the clicking around. You have to just sit there and let them figure it out. No hints and no hurrying along. Patients is very important. Really your only job is to ensure the keep moving along and keep talking aloud, which is harder than it looks.
Since I knew I would have a problem not blurting out hints and tips, I tried the 10 second rule. I didn't actually count, but I always waited after any question or if my participant got stuck. I didn't find that if I waited, they would trudge right along and figure it out. I didn't have to remind my participant of the rules and how I don't want to influence them. I wouldn't say I enjoyed it, but I can see after practicing a few times, you can get better at it.