Deep knowledge is important but I think I can add a few more thoughts:

- DON'T 'make-stuff-up' If you get a tricky question about something you don't know, just say you don't know - you will screw up and get caught out when you get it wrong. No one knows everything.

- DO only speak on subjects that you have a passion about I believe that this is very important, and the difference between a good talk, a bad talk, and a transforming talk.