Mine was pretty relaxed and both of my interviewers were really nice! It was a standard interview. Just be prepared to talk about why dentistry, why Penn, why you are a good match and why you're interested, where you see yourself in the future, talk about an extra curricular or two that you really enjoyed, strengths, weaknesses, something you overcame, what you do in your free time, issues in dentistry like weaknesses in the field, etc. These should be easy for anyone to answer especially if you've had interviews already! I didn't feel like I was being grilled at any point.