The viva isn’t the same as any regular day in your PhD, but it’s not so different either. Not really.
Do you need to be prepared? Yes.
Do you need to know what to expect from the viva? Yes.
But you also need to be a capable researcher. You need to be knowledgeable about your field and your research. You need to be ready to bring your best to the day’s work.
Isn’t that business as usual for you?
You need to prepare for the viva, but you also need all of the things about you that you would ordinarily have every day of your PhD. Preparation takes a little time and a little work; being ready takes a lot more of things you already have.