Certainty
You can’t know in advance what questions you’ll be asked at your viva. You can’t know how long it will be. You can’t know exactly what opinions your examiners will hold. A viva experience happens based on a range of expectations, combined with the regulations of an institution and the individual thesis and candidate.
And with all of that true, there’s still a lot you can be certain about for your viva.
You can be certain that your examiners have prepared. You can be certain that their questions, whatever they are, are all carefully considered. You can be certain of who your examiners are before your viva by researching them, if needed. You can be certain of the range of expectations because so many people have shared their viva stories.
And you can be certain of who you are, what you’ve done and what you know. You can be certain that you are ready.