You will be in a room with other applicants, you will fill out a lot of paperwork on top of the stuff that you had to bring filled out and go through each step.
You will be fingerprinted and checked that all your forms are in order. This is very boring and took a very long time, there were about 15 guys in our group and this process took almost four hours.
Make sure you have your selective service number printed out, it IS mandatory!
Than you will be interviewed 1 on 1 by a local Postmaster, there were numerous of them. The actual interview was very pleasant. He gave me what seemed like a no BS explanation of the job and what it entails. We spent very little time on the usual generic interview questions and had an actual conversation. They seem to care more about your job history and how you ended up applying to the USPS than anything else. All in all the actual interview was good. The paperwork process not so much. Four hours is a long time to sit around before getting the actual interview.