You should know the answer to "Well, why did you apply to OUR school?" before you get off the plane. 😉
I think that's one of the most important questions they ask, because they want to know that you've thought about your personal and professional goals, and where you best "fit" at medical school. Where can you be a part of the community, get the best education for what you want, have opportunities to do what you want, develop the skills you think are most important for a physician to have, etc.
Generally, you should know why you want to be a physician--inside and out. Personal experiences you've had that shaped your life, helped you make the decisions, taught you about people, life, and your own desire to help others in the medical way.
If you know yourself well, you'll portray yourself well to the adcoms, and they'll be impressed that you are a genuine, thoughtful, understanding, self-aware, well-rounded applicant. I think at the end of the day, that's all they're looking for in the interview. A so-so one can't totally break you at a school where your stats are very competitive; conversely, it can't be the only deciding factor if you're not statistically competitive (a GREAT interview can't get you into Harvard if you just don't have the grades; there's only so much it can do for you, etc.).
Practice explaining who you are, what sets you apart, and why you want to pursue medicine--talk to anyone who'll listen. Friends, a spouse, coworkers, etc.
Good luck to you!