Two words that really summarise what you need to do in the viva.
Respond: you don’t have to have an answer for every question. You may or may not have an idea or opinion that’s fully formed. The viva is a discussion, not an interview, not a quiz. You respond to questions, offering what you can…
Well: …and it has to be your best thinking in that moment. Whatever you remember, know or can piece together needs to be shared clearly. You need to pause and think about what your examiners are really asking, so that you can meet the expectations of their question or comment.
Respond well. Pause, think, speak.