I had this experience at one interview last year. I would say know what you said in your personal statement and why you said it. Anything you said in it, be prepared to back it up. Also rethink some of the important experiences you had on work experience and what they helped you understand/made you think about. I think there will be some questions that you really have no idea about, for example I was asked the difference between managing and leading.
I don't actually understand the point in these interviews. They are trying to score a subjective interview objectively. I think they may as well give you a pen and paper and ask you to write down the answers. I think it might make it easier for them to pick, but I'm not sure it aids them in picking the best candidate.
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