Northwestern is about the only school where interviewing is common practice. Vanderbilt makes them available to applicants and it's highly recommended that you do it. However, it's not as necessary as at NU. Besides that several other schools will request a phone or on-campus interview from a small group of applicants. I believe Georgetown will ask for a phone interview and maybe Cornell will do an on-campus interview with local applicants. In short, interviews are fairly rare. That said, if you apply to a school and then visit the place in order to sit in on a class and tour the campus; most of the time you will be granted a Q&A with an adcom. Most schools explicitly state thaat this is only an info session and will not affect your application. At many schools, you are being evaluated. Thus, it is an unofficial interview.