It's nothing more than what you have already been through. Just be confident and intelligble. These people aren't necessarily looking for the brightest people, just ones that will fit their program well and who can be professional and pleasant to work with, and who can successfully complete their program. I've heard of some sites quizzing you on case scenarios or diseases, but that I feel is not the norm and something I have not encountered.
If you have good references and are confident in your current clinical practice, know what you want, and can be professional/organized, you'll be fine. Someone may be/"fit" better, but you will probably not be a stinker.