When you are given a responsibility to lead or be in a position where you have to be in front of an audience, it is your utmost responsibility to deliver your very best. Not doing so is a big disservice to them. So it does not matter if you are tired or under the weather or have a million things running through your mind. As long as someone else is giving their attention to you, which in essence is their precious time, you better deliver. Think of it this way, how would you feel if the role is reversed and the person that you are giving your time to is not giving their fullest? I doubt you would even consider that a time well spent. Likewise.