At the viva you have to engage with the questions and comments of your examiners. You have to be a good participant in the discussion that rises from your thesis, the questions and what unfolds naturally.
You don’t have to be extra-smart or super-relaxed. They’re not looking for you to take on a persona. You don’t have to have an answer for everything or speak in a certain way.
Your examiners want to hear what you think and see what you know. Your role in the viva is to respond. You don’t have to be anything you’re not.