As Ksorelle said, the bulk of the interview will be discussing the topics listed in the interview request.
In addition to that, they'll also ask about your motivations for being interested in PC, that sector, and that country. They'll ask if you have any concerns related to various issues (status of women, lack of modern conveniences, isolation, dietary changes, etc), and they'll confirm that you understand the drug/alcohol policy. There might be some country specific questions, as well. For instance, I was asked if I was comfortable living with a host family for the duration of service (required in Cambodia), and was asked about my ability to ride a bike (Cambodia PCVs main mode of transportation.
After all that, your PO will more thoroughly describe the position you're interviewing for, and ask if you have any questions (prepare a long list, because most will be addressed before you have a chance to ask).
It seems stressful, but as long as you're prepared to respond to the things listed in your interview request, you should be fine.