My supervisor said I should choose words carefully
1) research terms can be specific and
2) It's better to say you are investigating the proposed theory that..., rather than saying you are trying to prove that...
A prof told me I should say I don't know, be confident if I do know, and give short answers rather than floundering about so the examiner can steer it towards what I do know as EVERYONE has gaps. The examiner can ask you more detail if needed. Some questions will be the examiner trying to understand new work, not trying to trip you up.
I found I got through 1.5 chapters in 2-3 hours practicing with my supervisor so it may not be as comprehensive as you think. It could take days to go over a viva fully.
Top tip-if you pick a v busy examiner or one travelling a fair way they will have limited time and need to get back and give you a short viva
My viva is this afternoon.