What can you say about a difficult customer you encountered recently? Why was he difficult? What did you do to satisfy the customer?
Some customers have unreasonable demands. When did you last encounter such a customer? Why did you find his demands unreasonable? What did you do eventually?
Which traits are important in order to interact well with customers? To what extent do you have those traits? To what extent could you train yourself to obtain these characteristics?
Could you describe a situation in which you dealt with a customer ineffectively?
When was the last time your boss criticized the way you deal with clients? How did you earn this criticism? How did you respond?