Do some research into the topic and make sure you read some of the recent literature, especially if they have recommended you read certain papers. I asked the supervisor if he recommended any reading before mine and got given copies of 20 papers! But I doubt most supervisors would do that!
Most of all the interviewers just want to see that you can cope with a PhD and indeed that you know what a PhD entails, so say you are keen to do conferences, write papers, learn new skills etc etc. Your work experience will be very useful so make a point of mentioning skills you have learned. I had an interview with a panel of five people, which seems scary but in the end they were all very friendly, and even helped me answer questions when I got stuck on words!
So just try to be calm and confident and have a clear answer in your head to the question 'why do you want to do a PhD?' as that's probably the most important one!
Good luck!