You guys are probably overthinking it if you aren't excited about interviews.
Interview excitement is really like a U-shaped curve....you get the invite (awesome, very excited), kind of dies down but when you finally get to the med school the night before, stay with host etc. your excitement should peak once again. You just ride that high into the interview and you'll be fine. If you're nervous, do a mock interview or take a nap/ play video games. Most would recommend the former, but knowing some pre-meds, the latter is far far more beneficial.
No one expects you to be excited every day between your invite and your actual interview, so don't worry about it. In my opinion, if you really want to make sure that you're excited for an interview, go get a camera, arrive at the med school the day before, and start taking pictures of the med school/undergrad/city and you'll probably start getting excited.
Regarding your initial query concerning the acceptance rate of interviewees, you are right that an interview does not guarantee an acceptance. But your chances just went up considerably, and I mean extremely, if you got an interview. Some schools accept upwards of 50%, some accept only about 10-20%, but hell, 10-20% chance at any school is still a much better chance than what you had when you were filling out your AMCAS.
If you have one interview, I can understand being nervous and perhaps not even excited because you fear that you might not make it (still though, its all relative, and your odds just went up by a lot). But if you have multiple interviews (especially anything more than 3 interviews), chances are that some school will like you and you'll get an acceptance, so you should probably relax and just make sure you appear enthused/excited during the interview and hope for the best.
You have to remember that you will only be attended one medical school, regardless of how many interviews you have. Don't expect to get into every school you interview at, but you should definitely walk in with the attitude that you're willing to prove why you're worthy of an acceptance at every school you interview at.
Expect the worst and hope for the best. If you're nervous, take a walk/hang out with friends and then do a mock interview, or read up on your research or on health care reform/ethics. Whatever you do, make it productive. That's what's most important, and when you actually are in medical school, having that attitude will do you a lot more good than worrying about the unknown and the uncontrollable.
-Dr. P.