My, this talk of vivas (vivae?) has taken me back to mine. It was over 40 years ago, with my research supervisor and the external examiner. Fortunately for me, the latter was enthralled by my work, so much so that, after asking me a few questions, he rushed over to the blackboard and spent the remaining half hour scrawling enthusiastically with further ideas. I didn't understand any of his stuff, but used my 'nodding intelligently' skill to the max! Such a relief.