I am assuming that UNTHSC is the University of Tennessee Health Sciences Campus. If so, I interviewed there back in November. As far as I know they are accredited and that should not be a problem. You can always double check on the PTCAS website by finding the schools profile. It will tell you everything you need to know about their accreditation.
As for the interview, I wouldn't sweat it too much. They set you up in a waiting room and then a student will come grab you when it is your turn. There will be three people conducting the interview. For me it was a student (2nd year), a faculty member, and an outside clinician that helps the program. From there they asked me to tell my story - about myself and how I got interested in PT and why UNTHSC. Then they proceeded to ask me some questions. From what I recall, none of them were difficult. They were more conversational than anything.
Make sure you are prepared to answer a question on Vision 20/20 or at least work it into your answers. They might ask you about current healthcare issues. I used Medicare caps and discussed how it affects the patient, the facility, and the therapist. I emphasized the affect it can have on the quality of care for the patient and their success in rehab.
Beyond that I don't think they ask you sandwich, animal, or any other off the wall questions. It never hurts to be able to articulate your strengths and weaknesses. They didn't ask me, but it is one of the harder questions for people to answer well.
Make sure you have a few questions to ask them, many will come up through conversation. But have a few on stand-by. Try to avoid saying that your reason for PT is that you want to help people. It is a helping people profession so it is looked at as a pre-requisite not a motivating factor (even if it is one). Make sure when you are speaking that you make eye contact and that you remember that there are three people at the table so speak to all of them when you respond.
With all of that said don't be too worried about it. In my experience, if a school invites you to interview they have a strong feeling that they want you. The interview is just to check to make sure you aren't a total waste of space when it comes to interpersonal interactions.
I hope this helps. If you have any more questions throw them up and I will try to get back to you before your interview.