I don't have a most difficult or unexpected question but the format of the interview is what they call SARI: Situation, Action, Result, Interesting fact. Any question asked goes something like, "describe a situation when you had to deal with a difficult customer, or employee". Answer it in less than a couple minutes. Describe the situation, feel free to use names, they won't know these people, state what you did, what the other person did, and for the interesting fact that is basically what they take out of the situation and could be a follow up question based on something you said. It is easier to state something like, " if I had to do it again, I would probably approach it this way..."