Vivas are governed by regulations. There are over 100 universities in the UK, each with their own set of rules for thesis examination – but these rules are all very similar in purpose.
Vivas are mostly conducted by academics. While there are typically two examiners in any viva they have colleagues who they talk to. Ideas of what makes a viva “good” or “right” are passed around.
This leads to cultures of thesis examination.
Culture can be specific to individual departments. Academics can have the idea that a certain length of viva is desirable, a certain focus, a certain structure and so on.
So: there are rules for what happens, ideas for what is right and these lead to patterns of experience by candidates. Viva stories describe exams tending to be a certain length, beginning with similar questions, and so on.
Patterns of experience, if passed on, give rise to useful patterns of expectation.
You can’t know exactly what will happen at your viva. Every viva will be unique, but if you ask the right people the right questions you can get a good idea of what to expect – and then prepare accordingly.
Talk to your supervisors, friends and colleagues to find out more of what vivas are like in your department. Understand the pattern of what happens at the viva and you’ll know what you need to do to be ready.