The questions your examiners ask will obviously be very specific to your thesis and anticipating them is a big part of your specific viva preparation. There are a few things that are likely to crop up more often than not, though.
Here are some example viva questions, along with some tips for answering them well.
"Why did you choose this PhD project?" / "What interested you most about this topic?"
This is a classic icebreaker: it's an invitation to speak generally and positively about your work. As well as being a fairly easy question to answer (after all, there must be at least something you enjoyed about your PhD) this should also help you channel your passion and enthusiasm for your research as the viva gets going. It's really quite a nice opener, if you think about it.
What it isn't is an attempt to catch you out or a challenge to impress the examiners by coming up with something exceptionally clever. Just be honest and speak from the heart.
"What was the most challenging part of the project?"
This probably won't be the first question you're asked, but it might also come up early in the viva as the examiners ease you into talking about your project. It doesn't mean that they think your PhD is flawed, by the way. All research involves overcoming obstacles. This is an invitation to talk about how you did that and reflect on the practicalities of your project.
Be honest, but remember that part of your goal in the viva is to back up the validity and coherence of your findings. It's not a good idea to suggest that your project met with critical flaws or limitations, unless you can also explain how you adjusted your methodology or objectives to achieve success in spite of them.
You should also avoid flippant or throwaway responses (that goes for the viva as a whole, really). Don't say that the hardest part was getting the thesis printed and bound, even if, a month ago, it really felt like it was.
"What is the original contribution to knowledge made by this thesis?"
This question is highly likely to come up at some point in the viva and it's one you absolutely must have a clear answer for. You should be able to explain in one or two sentences what your contribution is, how it's original and why it matters.
You don't need to be clever or extravagant. In fact, your answer may well just paraphrase a relevant part of the abstract or introduction from your thesis itself. But your examiners really do need to be confident that you can answer this one.
Some examiners might not be so explicit or direct in asking this, so be on the lookout for questions like "why is this PhD important?", "why was this project worth completing?", "what were your main findings?" or "why does this research matter?". If you hear any of those, it's time to deploy the original contribution answer.
"Why did you include / exclude X?"
All doctoral projects need to be selective about what they can and can't include, and successful PhD students need to set boundaries for their research. At some point your examiners will probably want to see the logic behind yours.
Be confident and own your decisions. If there was a particular topic or approach you didn't include, then give your reasons for that.
Remember that there are lots of reasons why something might not make the cut for a PhD and the examiners aren't trying to catch you out. They don't even need to agree 100% with your decisions, but they do need to hear that you had credible reasons for making them.
It may be that there wasn't space to cover everything (in which case you should justify prioritising the material you did include). Or perhaps you felt that there was already sufficient scholarship related to a particular source or concept and your aim was to take the field in a different direction (this is a very good answer, if you can make it convincingly).
"If you were to repeat this project, what would you do differently?"
This question (or one like it) may come towards the end of the viva as you reflect on the project as a whole.
Again, the aim isn't to try and undermine your thesis, but rather to see whether you can constructively critique your own work and approaches. Or, to put it another way, have you learned anything from the experience of doing a PhD? You should have. After all, a doctorate is partly about learning to become an effective researcher and mistakes are a great thing to learn from.
In any case, this shouldn't be too hard to answer. There are likely to be all sorts of things you would do differently in future: from adopting different approaches or directions sooner, to heading off blind alleys or methodological mistakes.
"What do you think the next steps might be for this research?"
Relax, your examiners aren't expecting you to dive straight into another PhD. But they may want to hear where you would take this research next, or what you think other scholars could do to build on your findings. After all, part of the value in a new contribution to your field should lie in what it makes possible, as well as what it is.
It's best to be modest and realistic here, rather than making sweeping claims for how your findings will allow other researchers to reinvent the wheel (unless you have actually come up with a new technique for designing wheels, in which case, go ahead).
"Do you have any questions or comments for us?"
Your examiners will probably end the viva by asking if you'd like to ask them any questions, or say anything else about your thesis. This might seem a bit odd, but it's actually a helpful way for you to revisit or clarify any of your earlier answers.
For example, you might like to acknowledge a specific critique and reiterate your reasons for believing the thesis to be valid in spite of it. Or you might want to confirm that the examiners understood what you meant at a particular point in the previous discussion.
It's not a good idea to try and rehash large chunks of the viva here, but it's fine to pick out one or two things and be assertive. This demonstrates your confidence and commitment.
Equally, you can take the opportunity to ask the examiner's opinions on areas of the thesis that haven't come up, if you wish. This is fine, provided you're confident in those sections and comfortable discussing them.