I had once a few weeks ago. Only general questions, nothing about my major or academic backgraound. The guy, who gave me the interview, was pretty nice. Enjoy your interview.
I got the offer a few days before, but I do not decide yet. I have 3 now. I need to do more research on each program and its job potentials.
Don't worry about the interview. They give you interview only to test your English skill. It shoud be around 15 minutes. Read your essay and resume again. Normally, he will ask some questions from your application materials.