Just be clear about your reasons for medicine and honest with your answers to their questions. They will ask similar questions to a med school interview, most likely, so you can apply any advice you see on the pre-allo forum and apply it to this interview. Mine also lasted longer than the one med school interview I have had so far.
Also make sure you have good answers, not excuses per se, for the deficiencies in your application. Since the purpose of attending a postbac is to make up for these, you need not feel like you have to make an excuse but you should be able to show why you have changed, if the deficiencies are bad grades, or why medicine now, if your deficiency is a lack of the science pre-reqs.
Good luck and stay calm. I actually didn't prepare one bit for this interview, I came from a field where I never interviewed...so I was a bit ignorant. However, I had thought a good deal on my own about these questions, so it went really well.