This question "What should I expect in the MD/PhD interview" comes up very frequently. Any of the comprehensive FAQs (see my sticky) or a search will easily cover it. That being said, I know you're nervous so I'll make you a quick response.
To summarise, the interview is not "two days of grilling", but rather 1 hour of grilling and 47 hours of selling you their program. I'll explain. There are two types of interviews:
1) Interview with the director/asst. director/adcom or with a committee. This is the interview that really matters. There's usually only 1 or 2 of these per visit. They will ask you the usual questions, "Why MD/PhD? Why not MD alone? Why not PhD alone? Tell me/us about your research experience" You can expect this to be the real interview. You shouldn't expect really any major curveballs. All of the talk will be about your app, your research, etc. From your MDApplicants profile, it looks like you're interviewing at UMaryland first. That was my first interview also. If it's like what it was 5 years ago (!), the committee interview will be what really counts.
2) Interview with whatever researchers they set you up with. This is to sell you on the school. They will typically do 95% of the talking. You don't prepare for these interviews, as you will rarely get a word in edgewise. The almost sole purpose is to sell you the school. Yes, they may be able to write evaluation forms about you, so you want to stay awake (HARDEST PART!) and try to form intelligent questions if you ever get room to ask any.
Sure, there may be exceptions, but this is usually the case. Don't focus on the weird reports you occasionally get. MD/PhD interviews tend to be more straightfoward than MD interviews, but that being said, every applicant I have told "STOP WORRYING!" has come back and said "Neuronix, you're right." Trust me.
To answer your questions specifically:
You were a non-science major. So what? If you have the research experience, you have the research experience. They can theoretically ask you any level of detail about your work. Usually they're trying to get an impression of you. "Tell me about your research" is your opportunity to summarize what you've done, and your responses to their questions will probably give them their gut feeling about you, how well you understand what you're doing, and how insightful you might be in the future. These questions are usually one of: how does/could this apply to medicine? What do you want to do next? Where do you see this project going? Or some nit-picky details about the actual science you were doing. The level of detail is in many ways determined by whether or not the interviewer knows anything about the research you do. Sometimes you get someone who does what you do and you get really in depth, and sometimes you get someone random and the questions are really softball. Don't be worried. If you know your stuff, you know your stuff. Also, they recognize that you are an undergrad--not a post-doc. Nobody is going to say "You were a music major? WTF are you doing here?" They wouldn't bother interviewing you if they felt that way. They might ask you about it, you tell them whatever reasons you have for getting to where you are now, and you move on. It's not a big deal.
