If your application is in, and the college hasn't called to schedule an interview, should you worry?
The answer here is no. Most colleges don't require interviews, and those that do are very good about contacting you to schedule them. Most students breathe a sigh of relief when they hear this, and I don't know why -- after all, why wouldn't you want someone at the college of your dreams to hear about your dreams?
In any case, interviews come in three different varieties: On campus, near your home, or via technology (phone, Skype, etc.) Since most on-campus interview times are taken by now, it's likely the local alumni office will call to set up the meeting. If they leave a message for you, call back right away, and work out a time and date. If this is a face-to-face interview, you need to arrange a place to meet, which should be a public place that offers relative privacy (like a coffee shop).
The day before the interview, make sure you confirm the details. Plan on dressing on the dressy side of casual (really remember this if it's a Skype interview), and arrive or dial-in early -- as in, five minutes before the interview starts. If you get lost on the way, CALL if you can. Being lost is not grounds for being denied admission; being gracious about getting lost is the thoughtful thing to do, and demonstrates your humanity to the interviewer.
The key to a successful interview is to just be yourself -- really. The interviewer will probably ask you open-ended questions -- questions where you will have to supply more than a "yes" or "no" answer ("Why are you interested in our college?" "What's the most interesting thing you've done?" or the famous "Tell us about yourself.")
Your answers should be complete, and of a good length -- about a minute or so. Keep the rules of a good college essay in mind -- answer the question, show warmth, humor, intellect, and grace (easy on the humor -- don't force a funny moment), and show the personal side of yourself without getting too personal. If you want to relate a story you told as part of your essay, that's fine, but don't make it the entire answer to a question -- this is a chance to add to your application, not duplicate it.
You may be asked if you have any questions. It's good to think about one or two in advance, but if you think of one on the spot, that's great too. These questions should require answers that aren't in the college catalog or brochure -- remember, the quality of the question can show the thought and interest you've put in to investigating the college. If your interviewer is an alumnus of the school, ask them about their perceptions of the college -- what it was like to go there, and how it helped them in life. The answer you get is just their opinion, but it's still good information to have as you consider the best college to attend.
When the interview is over, stand up, thank the interviewer, shake hands, and off you go (or say goodbye, and remember to sign off your Skype account). It is exceptionally good form to call or e-mail the interviewer the next day to thank them for the interview, and invite them to please contact you if they have any other questions. Most students don't do this, and while that isn't held against you, it's a nice touch -- and since making the most out of college means going the extra mile, this is perfect practice.