You need to convince the prospective employer of three things: I can do the job. Here is why I can do the job. I want the job.
Prepare a list of questions.
Write your questions down and take them to your interview. Intelligent questions indicate genuine interest and preparation. Ask the questions of everyone with whom you interview. Different perspectives will give you a better understanding of the practice or institution.
Collect the facts on your interview.
As Adventures in Medicine writes in their article Preparing for an Outside Interview "Employers invest a lot of time and money in candidate interviews, so do your best to avoid the need for a second visit. Be prepared to gather all of the facts about the opportunity, the practice and the community on your first interview."
Ask your interviewer to detail the duties of the position.
Early in the interview ask to have the duties of the position clarified so that you can relate your background and skills with those duties in mind.
What is the next step? Never leave the interview without a clear understanding of the next step. Will they contact you either way, or will you only be contacted if an offer is extended? In what time frame will they make a decision? Will they contact you directly or through a recruiter? Is there someone whom you may call if by the deadline you have not been informed of his or her decision?