There’s a rule of thumb for the viva some examiners have mentioned to me:
“The first question will be easy; the last question might not be.”
There’s no trick to the first part of the statement. Examiners want the viva to go as well as it can. The first question is likely to be something you’ve thought about or could realistically expect (like how you got interested in your topic). The intention is to help get past the awkwardness and nerves of being there and get down to business.
There’s no trick to the second part either. You might get tricky questions in the viva. You might face criticisms of your work. You might find the discussion leads to a tough debate. Given the nature of what you’ve done and what the viva is for, it’s reasonable to expect the odd difficult question, particularly near the end.
It’s unreasonable to think that every question will be hard though. Expect the viva will start well. Expect your examiners will ask tough but fair questions of a talented person.