As preparation for the viva read three recent papers by your internal examiner. Read three recent papers by your external examiner.

Look for links between your work and their work. Look for connections between your interests, methods and the kinds of questions you ask.

When you find connections you find ideas that are worth thinking about so you can explore them well in the viva. If you discover that your work is not closely connected then you can think before the viva about how you can share your ideas effectively.

Knowing about the links between your work helps. Knowing if the links aren’t there helps too.

You have to look for the links.