Yeah, it's a bit embarrassing asking the usual questions to people that you know - and even to strangers. I try to give people a chance to talk about scenarios they have really dealt with, rather than those awful questions about strengths and weaknesses.
How about:
Can you describe a time when you have had to deal with a difficult client/call, and how you handled it?
How do you handle it when you come up against a problem that you don't immidiately know how to tackle?
What do you think customer service means in the context of this job?
How do you keep your clients/customers calm when they are anxious or frustrated because of the problem they are encountering?