Never anticipate the point or joke of an anecdote told in your presence. If you have heard the story before, it may be new to others, and the narrator should always be allowed to finish it in his own words. To take any sentence from the mouth of another person, before he has time to utter it, is the height of ill-breeding. Avoid it carefully.
Be careful always to speak in a distinct, clear voice; at the same time avoid talking too loudly, there is a happy medium between mumbling and screaming. Strive to attain it.
This particular quote is from page 14 of the The Ladies’ Book of Etiquette, and Manual of Politeness (1873), source here. Warning to all: avoid the “height of ill-breeding” in the new year! And, happy new year!