Never does a man portray his own character more vividly than in his manner of portraying another’s.
Jean Paul Richter (1763-1825) German writer, art historian, philosopher, littérateur [Johann Paul Friedrich Richter; pseud. Jean Paul]
Titan: A Romance, Jubilee 28, cycle 110 (1803) [tr. Brooks (1864)]
(Source)
Alternate translation: "A man never reveals his character more vividly than when portraying the character of another."