My parents were interviewed together by a friendly officer. They were interviewed about 2.5 hours after their scheduled appointment (8am).
The interview lasted around 20 minutes. They were asked to provide a new police certificate (which we fortunately had), marriage certificate, their birth certificates, and my birth certificate. They were asked about how I got to the USA, information on my employment history, where my sister lives and why they decided to relocate to the USA and not where my sister lives.
Besides the wait, they had a very pleasant experience and they are extremely grateful that they have been approved.