Rehearsal for the viva is essential because it simulates some of the aspects of being there. If you’re nervous about how you will respond, what will come up or what it will feel like to be there then you can’t do better than rehearse in some way.
A mock viva is the best way to simulate the viva experience. Questions from experienced academics in a relevant field or disciplines. Time to think and be in a viva-like environment. Facing uncertainty of what the next question or opinion will be. You can practise what you will do and get a sense of how you might feel.
A mock viva, a simulation, however polished, can’t be as accurate or as good as the real viva. How will it differ from the real thing? You can’t know beforehand. You can simply be better prepared for talking to your examiners.