Every viva is unique, but there are enough common experiences that you can help yourself be ready.
It’s like walking through the countryside and you don’t quite know where your destination is. Your travelling companion asks where you’re going and you wave your hand vaguely and say, “Somewhere over there!”
Expectations for the viva give you a compass. Knowledge about viva lengths helps you to prepare yourself for the effort. Understanding the purpose behind questions raises confidence for responding. Expectations help give you direction even if the final destination is a little uncertain.
The more you know about what to expect from the viva generally, the more you can help yourself be ready for your viva particularly.