In British Universities a ‘viva’ refers to a PhD exam, contrary in the US this is referred to as ‘defense’. Either way, an ideal PhD student should be able to verbalize their thoughts freely and hence you have an oral exam. Another reason being is to check that you are the architect of the master piece.
During a viva I recommend a student follows some of the following tips (some but not all):
1) Don’t ever apologize for not knowing the answer of a question put forward to you by your examiner. This is very unprofessional and so why not just bluntly say you do not know the answer of the question and justify your reasoning; maybe you were not working closely in that framework?
2) Don’t avoid spelling out your qualities or research contributions. What better time could there be than this, but don’t lose the plot and start acting as know-it-all, unless you can really stand on your feet.
3) Don’t ever say explicitly that you are very nervous (NOT ‘excited’ as that has a positive connotation!)- try to be relaxed and as professional as possible.
4) Be able to internalize certain terminology or findings with wider concepts.
I’ve attached a file with some more tips…Viva