For me, I thought a lot about the answers to the questions "tell me about yourself" and "why X field" and came up with enough talking points that I could talk for the entire interview (20-25 minutes) to answer those two questions. I kept this rough framework in mind going into every interview, and when I got a question I didn't have an amazing answer to ("tell me about a personal weakness," "tell me about an interesting patient," "what's a challenge you faced," etc) I answered that question and simultaneously worked to steer the conversation back to my mental schematic of "tell me about yourself/why this field" for which I felt like I had a strong answer. For example, if asked to talk about an interesting patient, I picked two: one from which I could transition into why I liked the field, and then one from which I could transition into talking about particular qualities/aspects of myself. It's a nice long answer, ultimately reducing the duration of the awkward "so, what questions do you have for me?" period of the interview. Obviously, I only did this if it felt organic, and if I thought I'd have to force things to continue, I shut my mouth and waited for the next question.
For the actual answers, I thought about my traits I wanted interviewers to appreciate - I have a background and interests which are very uncommon among med students, I'm good at getting patients to like/trust me, and people like working with me - yet which are hard to quantify on paper, so thought about what I could say that might convey that. I wanted to come across as someone who was less impressive as just interesting to talk to, and so tried to focus on being conversive and viewing each interviewer as someone I already knew and didn't have to work to impress. Think about it this way: programs are working hard to sell you on what makes them unique - schedule, facilities, didactics, whatever - so you should focus on the same. Every applicant has Step scores and grades, so show them something only you have.
I've gone into my interviews with the mindset that I had impressed the admissions committee enough to get an interview, so I didn't have to worry about defending myself or pleading my case; rather, I'd focus on showing them I was someone who'd be fun to work with. Just like you, I knew I'd be interviewing with applicants who are stronger on paper than I am, but I've tried to view each interview not as a challenge but as an opportunity to say "forget the numbers, let's just chat."
Finally: smile, sit up straight, dress well, make sure your suit jacket is clean, and act like you're pumped to be there (because you should be).