When it comes to the MMI, there is one simple rule. Be yourself. If you are interviewing, then the school has already decided that you are someone who can handle the academics and are someone who has something to offer the school in terms of your background. The MMI is about seeing if you will fit in and thrive with the other personalities.
If you come in acting like someone you are not, then either they will pick up on that and hand you a rejection or two things will happen. First, you will get here next year and not fit in with your classmates, which will make medical school just about the worst experience of your life. And, you will be that student that faculty, staff, and administrators are thinking about when they talk about the few students who manage to slip through the cracks to gain an acceptance they should not have.
So, again, just be yourself. That way, if you get in, you will know you belong here. And if you don't, then you know you would not have been happy here anyway.