Are there usually interviews for postdoc positions? I applied for postdoc positions and I received a few interview offers. I thought not many institution have interview process for postdoc positions and they give offer only based on application material. Do I have false information?
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