I interviewed there 4 years ago and was accepted. It was a really nice interview set up, much like a medical school interview that we are all now going through.
First, I had a lunch interview with a medical student in the cafeteria. It was pretty laid back and we just chatted for an hour or so. He showed me around some of the facilities.
Then I had a faculty interview that was pretty chill too. We talked about medical issues and ethical dilemmas but nothing really hardcore. I really enjoyed my visit to Northwestern.
My best advice is to go prepared to ask some questions about the medical school and to give good reasons for why you are so intent on pursuing a medical career. Being prepared to answer certain questions is the best thing you can do before you go. Best of luck.