When I interviewed for my last job (about 3 months ago), I approached it in much the same way as I approached an irate end-user from my tech support days. Do your best to answer their questions, keep your calm, and always try to convey that you care about addressing their problem thoroughly as soon and as well as possible. You may not be able to slamdunk solve the matter immediately upon request, but it will have your attention until the problem goes away.
I think this just backs up the idea of coming across as someone they want to deal with everyday, who will tackle the job cheerfully. Obviously, it helps to be remotely competent in that area. In my case, I had never worked with the particular systems they needed dealt with; however my background and my attitude were enough to get me in the door.
At any rate, best of luck to anyone who is searching. This is not a great time to be in that situation...