I had the interview with NVCP last week. The questions they asked of course open-ended type, such as "Why," How", etc..
As of the essay, you shouldn't stress out so much. It's 30 min on a topic that requires your organized thoughts. Don't write too long nor too short. Just relax, organize your thoughts, make them clear and consise, you'll be okay.
About the interview, just prepare with questions that ucsfpharmgirl8 posted a while ago. But believe me, what most interesting things you find out about NVCP is the "block system" and how it really works...90% is the passing rate for an exam!!!!!