Following are some of the commonly asked questions in a PhD viva. You should write down the answers to each of these questions. It would be great if such texts exist in your thesis, in which case you may refer the examiners to the relevant texts, but you must also be able to say (not read) them fluently.
The examiners have already read your thesis. What you present in this summary defines the agenda of the viva. You don't want the viva to focus on something that you are not a world expert in. So don't say anything that is not core to your thesis.
A PhD is judged by its substantiality and significance. Focus on:
This is the most important part of your thesis. The substantiality and significance of your thesis is judged by what you claim. It is a mistake to leave it to the examiners to draw their own conclusions. They rarely draw the same conclusions as your before the viva begins.
In Computer Science, the question may take the form of:
Common mistakes on contribution claims:
By definition, a PhD thesis reports original research (only when it is original would it be considered significant). You should know the subject better than anyone else in the world, including your supervisor and the examiners, who may have broader knowledge than you. You cannot expect others to have the same knowledge are you in the subject. You must be able to explain your work to non-experts. Even if the work is very technical, you must be able to explain it in layman terms.
Every examiner would try to related your work to something that they know. Besides, your thesis is judged by how significant your contributions are. To demonstrate significant of your work, you must be able to tell which works you are building on, which works compete with yours, and how is your work different from those other works.
A PhD thesis should be treated like a mathematical proof. Most proofs have assumptions. You must state clearly what assumptions you use in your thesis. Being unaware of the assumptions of your thesis may lead to examiners questioning your understanding of your own work.
All works have limitations. The examiners would like to know that you are aware of the limitations in your thesis.
Having limitations is natural. It would be a mistake to try to hide them. The examiners may have already spotted some limitations before they ask you this question. Being unaware of the limitations of your thesis may lead to examiners questioning your understanding of your own work.