I've been reading some posts, and people have mentioned mock vivas?
Is it your supervisor who does them for you?

Usually, yes. As to whether everyone has one, I am not sure, but those of us on here who did found it very useful.

I did, it was about a week before the real thing. I found it really useful. A couple of days before I also had a chat / session with someone who videoed me whilst they asked me some questions. Whilst they were not au fait with my topic, it was useful to see what I actually looked like when put on the spot. It was not as bad as I thought thankfully!

If you only do one thing to prepare before a real viva, have a mock one!! I'm a firm believer in them As for who acts as examiner, supervisors are the usual choice but anyone with the required level of knowledge, familiarity with your thesis and the time to help will do (other students, post-docs?). Good luck!