The Ass in the Lion's Skin

An ass, having put on a lion's skin, roamed about, frightening all the silly animals he met with. Seeing a fox, he tried to alarm him also. But Reynard, having heard his voice, said, "Well, to be sure! And I should have been frightened too, if I had not heard you bray."

They who assume a character that does not belong to them generally betray themselves.