What do the tutors want from an interview candidate?

The interviews are really a kind of mock-tutorial, an opportunity for the tutors to see how you interact with them in discussion in an academic setting, so essentially what it would be like to have you as a student. You're typically given a text to read for five minutes before you go in, and you'll discuss that text with them. Discussion about your school life/ gap year/ personal statement will be minimal. They're looking for your ability to think critically about a text and about a subject, and your ability to think on your feet and debate. Ideally, you'll go in with ideas and an argument about the text you're given, but don't be afraid to ask clarifying questions if there's anything you're unsure on - it's not an exam, it's a conversation. With regard to the discussion, there's a line to be drawn between holding your ground and being bullheaded - just like any student, you should have your own ideas and have faith in them, but equally be willing to learn from your tutors.