You don't always have to be extroverted to interview well. One of the things that helped me was asking the interviewer questions about the school which made me feel like there was less heat on me. Granted you will still have to answer questions about yourself (why dentistry and whatnot), so you should still practice but don't sound as if you've memorized a script.
And remember that the school is trying to sell their program to you just as much as you're trying to sell yourself as an applicant. At a lot of my interviews I usually spent less time answering questions about myself and more time asking questions and listening to what the interviewer had to say about the school, the field of dentistry, and even stuff about themselves, where they've traveled, how they got to where they are now, etc. And if you're interested in volunteering, teaching, research, or specialties and your interviewer happens to be involved in any of those, this is a great opportunity to ask how they got into that field. Being extroverted and personable can be a huge advantage for interviews, but if you're introverted you can also make being a good listener work in your favor.