Hi Cloud Nine, It's generally done by the senior but it could be done by anyone. I used to encourage my crews to do it so that they could could cope if they were ever in the position of having to do it without me being there. You never know what could happen in flight. Adding bits is generally frowned upon by the powers that be these days. It should be done seriously seems to be the view. All I can say is when I was flying if you added a bit of humour people actually took notice of the demo instead of ignoring it. Maybe some else could say what the current view actually is?