Just had mine three days ago. I was strangely calm before I went in and through the viva as well. Your supervisor is right. It is a discussion of your thesis amongst equals. And you know your thesis more intimately than anyone else so don't worry, the defending part of it will come naturally. I found that after nit-picking and thinking my thesis over thoroughly while writing it up, the viva itself was more like explaining my thoughts and concepts to the examiners who had read my thesis in a short time and wanted to know more. My most valuable tip is to not study the day before the viva. Choose a cut off time, like 2PM the day before the viva and go see a fun movie, play or visit friends (Don't get drunk). Get up early the next day, eat well, dress comfortably and be confident but don't bluff. Be nice and accept criticism if there is truth to it. The viva is not a demon.