I can’t say people never get tough questions in the viva.
I can’t say candidates never face examiners who are overly harsh.
I can’t say YOU won’t have a bad viva.
Because people do. Even though most people have a perfectly fine experience in the viva – tiring, draining, tricky but overall fine if not enjoyable – some people don’t. So what can you do?
Keep doing good work now. You can’t know in advance what your viva will be like exactly. Good or bad, right now you can put yourself on the path to be the best you in the viva. Someone who knows their stuff, someone who knows how to be a good researcher in their field, someone primed for confidence and able to answer questions about their research.
Bad viva? Not in your control. Good candidate? That’s all up to you.