For almost every school, you will interview on campus (Johns Hopkins, I believe, offers regional interviews, but they're the only medical school I heard of doing such). And it will take all day, so don't bother trying to schedule two interviews in the same day.
It is a very expensive process. I think when all was said and done, I spent $2500, not considering food expenses, to just apply to 9 schools and interview at 4. I'm already up over $1000 this cycle, and I still have one secondary to receive and haven't been invited to interview anywhere. You can cut costs somewhat by trying to schedule two interviews in the same region around the same date, and by staying with a student host (or family friend), rather than in a hotel.
Most professors will be ok with you missing a class here and there. Chances are, if you apply to 20 schools, you won't get interviews at all of them. Hopefully you'll get an early acceptance too, so you can pick and choose which interviews you want to go on. So, it might mean like 5 missed classes in any given class in a semester. I managed to schedule my interviews around classes for the most part (I had Fridays off, and two of my interviews were on Fridays), and when I was at risk of being delayed for a test, I let my professor know ahead of time. Most students will be applying to some sort of graduate school and professors tend to recognize this. Just be courteous about it.