The simplest way to describe your role in the viva is that you are there to engage with your examiners’ questions. Whatever the question is – easy, hard, expected, unknown, hoped-for or unwanted – engage with it. There’s space for you to ask your own, of course, but for the most part you play your part by responding to questions and being a full participant in the discussion.
Whatever they ask, think and respond. There’s useful prep that can help but keeping that thought in mind once you get to the viva can do a lot to prepare you.
You have one job. Engage and respond, whatever they ask.