There are two types of people who could possibly be your interviewer. The first is an alumnus of the school you are applying to. These interviews are less impactful than admissions committee interviews, but can still help or hurt your application. The alumni interviewer will probably interview many people from your area and will have an idea of what to expect when a student from “XYZ” school is the applicant. You should research the alumnus a little bit, so that you have an idea of what he or she likes
The second type of interviewer is an admissions officer. These are most likely to occur on campus, and are offered to a select number of applicants. These interviews are more formal than the alumni interviews and should focus more on why they should admit you rather than on why you should you consider attending the school.