The interview is an important part of the selection process. Basically the admissions commitee has decided you are capable of doing the work and would like to meet you, the person. Usually an interview could last from 30 to 60 minutes depending on how much they like OR dislike you.
First impressions are long lasting.
Check out dental careers on the ADA site. Ok, easier said then done. You are probably right. All I can say to you is practice, video tape yourself or even have a friend interview you. Well that depends.