Never seem wiser or more learned than the company you are with. Treat your knowledge like a watch and keep it hidden. Do not pull it out to count the hours, but give the time when you are asked.—Philip Stanhope, 4th Earl of Chesterfield (by Allan Ramsay; Public Domain)
Few things betray a person faster than their own hubris. Arrogance quickly reveals itself to the audience, but remains strangely hidden from the speaker.