There are several options:
a) Some schools required that YOU schedule an interview on campus before or after applying. If you failed to do so and they are interested in you, they may "remind" you of the requirement later during the app. season.
b) Some schools offer invitation-only interviews. Patterns vary widely, with some schools inviting only the applicants with high chances of admittance and other schools inviting virtually everyone.
c) Some schools may admit (or waitlist) a few candidates which they haven't interviewed. This practice was not unusual a few years back, though it seems to be very uncommon (if not completely abandoned) since recently.
Interviews can be: on campus with admissions staff, on campus with students, off campus with alumni, off campus with admissions staff, and on phone (typically with admissions staff, sometimes with alumni).
L.