Bank Job Interview please HELP!!!?

I have a job interview coming and it's for a bank Teller position. I know for a fact that they will ask me a question that I have had always problem answering it which is: Tell us a time that you went beyond everything to help a customer. I wnat to asnwer it in a way to impress them. Can anyone please tell me what would a great answer? What other question they might ask?