Here
https://www.enz.org/forum/showthread...076#post564076 is one thread where I outlined the basic situation. There are other old threads here.
https://www.google.com/search?client...ship+interview
The interviews last as long as the CO thinks necessary for him/her to get the information needed. Maybe 30 minutes to an hour is average?
Partners are usually interviewed separately. Because partnership visas don't require any proof of qualifications or work experience, they are the ones most often targeted by fraudsters, getting someone to pretend to be in a relationship. So INZ need to be sure that each pair are genuinely in a live-in relationship. Besides talking about how they met and the time-line of how they got together, I've heard of what may seem like trivial things, like one half of the couple being asked, 'Where does your partner keep his/her toothbrush?' or, 'Is your partner reading a book at the moment? What is it? Where has s/he left it?' or, 'What did you have for dinner last night? Who cooked what? Who poured out the drinks?' The point of these questions is that they may ALSO ask the other partner, to see if the replies match, i.e. if these people are genuinely living together.
Most usually, yes, it is the CO working the case who does the interviews.