Then the interview is likely to be about your partnership. The questions are usually such things as how you came to know each other first, when and why you started to go out together, what you had in common - the story of your couple, really. They can also ask you details about how you organize your domestic lives, such as who is responsible for this or that chore, and which of you shops, or pays for this or that. They can ask you about each other's likes and dislikes - food, colours, decor, music, pastimes, etc.. They can ask you about each other's family background - where the other person spent their childhood, the names of their siblings and childhood friends.
All this is because, with no qualifications or work experience to be investigated, partner-sponsored visas are the ones with the least formalities to go through, so they are the ones most targeted by fraudsters wanting to pay someone to lie for them to get them a visa. The questions are all the sort of thing that are quite straightforward for a genuine couple, but which would catch out two people who are actually strangers to one another.