Are there usually interviews for postdoc positions?
Assuming that you're a mathematician (seeing as how your question migrated from math.SE), I believe that you get interview if you applied for:


European institutions
tenure-track level positions at liberal arts colleges
lower-level research universities who want to gauge the applicant's interest (was the applicant applying to this position as a safety, or is he/she genuinely interested?)
(only sometimes true) positions where research and teaching are equally valued, or where teaching is valued more highly
(only sometimes true) if a particular researcher is hiring postdocs out of his/her own grant