Never does a man portray his own character more vividly than in his manner of portraying another’s.

[Nie zeichnet der Mensch den eignen Charakter schärfer als in seiner Manier, einen fremden zu zeichnen.]

Jean Paul Richter (1763-1825) German writer, art historian, philosopher, littérateur [Johann Paul Friedrich Richter; pseud. Jean Paul]

Titan, Jubilee 28, cycle 110 (1803) [tr. Brooks (1863)]
    (Source)

(Source (German)). Alternate translation:

A man never reveals his character more vividly than when portraying the character of another.
[E.g. (1960); E.g. (1962)]