How important is the college interview? The correct answer is, 'it depends.' There are over 2,400 four-year colleges and universities. Some put a great deal of weight on the interview; others, especially the most selective schools, rarely even conduct them, except through their alumni network. Of course, just when I'm ready to discount the importance of interviewing, a student, who was a true long shot at UCLA, gets admitted because of the strength of her interview. The interview can be a useful tool for students who are prepared.
The best preparation requires that you know the college. At the very least, this means you've gone to the website and read about its academics (especially in whatever major you might be interested in), reviewed core curriculum and requirements, and looked into whatever extracurricular offerings that you might consider should you gain admittance. If you've actually visited the campus, you want to recall your general impression of the school, and what qualities attracted you. The better you know the college, the more comfortable you'll be going into the interview.
Of equal importance is to know why the school interests you. Remember, you are attempting to make a match. The interview is your forum for explaining why your admission will benefit both you and the school. If you can clearly make your case, your interview should be considered successful (that is if you didn't spill your drink on the carpet, or insult the school or interviewer while you were explaining the dynamics of the 'match.'). By the way, many students ask me how many schools they should apply to. I always mention 8-9 should be the maximum (with a state school system, such as the University of California, counting as one). You can imagine how difficult and consuming the matchmaking process becomes when your list goes beyond this number.
Try to find out, ahead of time, who will be interviewing you. You will, in most cases, be interviewing with one of the following: an admissions officer, an alumnus, or, possibly, a current student. If you visit a target school, you might very well meet with the admissions office. Before you arrive, look over the list of the admissions officers (the staff should be listed on the school's website). Then do a Google search on each of them. If you can come up with recent photos, or find articles about them, you'll have a sense of who they are before you interview. Any familiarity will help you feel more relaxed with the interviewer (and knowing his or her interests before the interview will help you connect with him or her during it).
Anticipate the interview questions. You know, up front, that there are specific categories of questions your interviewer will ask (assuming you have a competent interviewer; if it's an admissions officer, it's safe to say she will be-if it's an alumnus or a student, you never know.) In any case, the questions will be about: academic life and interests, personal life, extracurricular activities, and college and future goals. Additionally, it's a given that at the end of the interview you'll be asked if you have further questions. Always come with 2 concluding questions.
Obviously, if you do mock interviews, you'll gain experience with the whole process. Using a video camera, you can evaluate your body language and answers. Yet, even if you don't have the time to meticulously prepare for whichever type of interviews await you, go to the interviews with a positive attitude, show you're committed to learning, try to relax and enjoy the interview, and be yourself. Good things come when the real you shines through.