It’s understandable that the nature of the viva could make a person worry. It’s understandable, given what any PhD candidate has to do to get to the viva, that the person being examined might be concerned or worry about how to do their best.
Or better than their best!
And it’s perfectly understandable why the thought of being asked this question or that question – or any question – might make someone feel nervous, concerned or stressed.
To simplify the situation, in the viva, questions are just questions. When you hear a “?” at the end of the sentence that’s your cue to talk. Your cue to talk about what you did, how you did it, what you know or what you think. It’s your cue to say something: to ask a question, to share a response, to say you need to think or to say you’re not sure.
Your examiners have to ask questions to find out what they need. You have to respond to those questions to try to meet those needs.
There are no good or bad questions, although it’s reasonable to expect challenging questions that you have to think about. It’s understandable for you to be nervous about being asked, but also reasonable to expect you to rise to the challenge of responding.