I don’t know there is much scope for candidates to lead in the viva. In the stories I have been told, I don’t hear tales of examiners sitting back and waiting for the candidate to direct questions, or steer them towards conclusions, or evade lines of discussion. It’s not for the candidate to dictate what happens (nor should it be up to examiners to dictate things either, of course). It’s for examiners to steer discussions, examiners to fairly ask questions, suggest ideas and examine the thesis and candidate.
As a candidate though, you can lead yourself. This isn’t a throwaway, simple, nice-sounding thing. You can lead. Set the tone for yourself. What do you expect? What standards are you aiming for? What direction do you want to go in as a researcher for the viva, and how are you going to get there? Ask yourself what “prepared” might feel like – then ask yourself what you are going to do to lead yourself towards feeling confident on the day?
You have to lead yourself. So what are you going to do?