DD had a couple of interviews already at schools that interview before exams.
The way I try to prepare is to be able to explore any question asked in depth and branch out. It seemed to have helped as that's exactly how the interview went a couple of times.
Basically, choose an answer that allows you to explore further. E.g. DD likes both science and art (favourite subject question). However, there isn't much she can say about art except that she likes doing it. If they started asking about her favourite artists, styles, museums, exhibitions, etc, she wouldn't have much to say. With science, on the other hand, she can talk for hours - about robotic surgery, DNA, viruses, black holes, space, cell structure, tadpoles, growing things, etc. Regarding art, we said keep it to one of the hobbies (she does an art club) and we did take her to Pollock exhibition.
Similarly, if there is a favourite book, we talked about why, any other books by the same author, what it is that you like about it, themes explored, etc. What are you reading now? etc.
Also, personalize - e.g. everybody will know about Nelson Mandela's death in current events or if you use him as a person you admire/find inspirational, that's pretty cliche. But DD's father is South African and took parts in protests against apartheid when at university there. So he talked to her about it, how it felt, why students took risks to do this, the split in attitudes between the boers and the rest, etc. Similarly, her grandmother did rowing big time at university and talked to DD about it so it was natural for her to say she'd like to take up rowing because her granny did it when asked about what you like about the school (with rowing schools) etc.
I think you get the gist - whatever question/answer it is, try to go deeper within that topic.
BTW, she is not interested in current affairs whatsoever, so we are going to stick to heads of state for most important countries, still struggling with chancellor (just asked her - it's Charles Osborne!, at least we are getting there with the last name). But she can talk with passion and insight about issues in society - education, taxes, immigration/emigration, budget deficit, etc. Ask her who is an education secretary and you get a blank!
So we'll stick to 2-3 key events in the last couple of months and leave it at that.