I have one as well! I've read in previous years it has been a group interview but the faculty member will ask some questions directly to each individual and that it doesn't feel really competitive against one another however I'm not sure if that is still the set up for it!
For basic things to be prepared for with interviews "...know about the profession, the school, why you want to go to this school, why PT, and why they should choose you. If you can, try to think of something unique about yourself for that last question..." (http://forums.studentdoctor.net/threads/mgh-interview.877411/)