My general advice for the interview is to be yourself! At this point you've proven you are academically fit to attend a medical school, and now they want to see what makes you different from all the other equally qualified applicants. I recommend going to the welcome event the night before if possible - it calmed my nerves and is a great way to meet students, faculty, and potential future classmates, and to get questions answered without the stress of interview day distracting you. Before each interview I attended (regardless of MMI or not), I brushed up on University of Washington's "ethics in medicine" page, and also searched online for some practice MMI scenarios. No way of knowing what you'll be asked, but I found it helpful to practice answering questions under time, how to organize my ideas, etc. Be fair, be open-minded, and be you.
There are lots of opportunities to identify a research mentor, and we have all of first semester to find one. Mentors can be through VTC, Carilion, or FBRI. Many mentors have pre-existing projects to join or projects they'd like to start soon, so there are many options for us, as well as the option to create your own project from scratch if you've got ideas. Our research deans are very willing to meet with you if you have questions or struggle to find a PI's project you like, and we had a few times where PIs came to speak to the class, and research fairs where we could "window shop" PIs. For the most part, PIs understand we have school to worry about too, and will let you be as overly or minimally involved as you want (within reason).