There are no right-for-everyone answers, only what is true *for you*. When anyone gets called for an interview about partnership it's because the officials are responsible for checking that the partnership is genuine, given that this class of visa is the one most often targeted by fraudsters paying somebody to lie for them, so in general, the CO will be making sure that the two people give roughly the same answers to things like how long they have known each other, how they met, etc., and they might ask about the other partner's background and family. These things are not difficult for genuine partners, but will catch out anyone who is faking a partnership just for one of them to get a visa.
Some of these interviews go into small matters like who does what domestic chores, how does it work, entertaining v. cooking, if you have people round for a meal, when in the day does the other one brush their teeth - again, real partners won't have any trouble, but fakers would be caught out.