Only animals who are below civilization and the angels who are beyond it can be sincere. Human beings are, necessarily, actors who cannot become something before they have first pretended to be it; and they can be divided, not into the hypocritical and the sincere, but into the sane who know they are acting and the mad who do not.

W. H. Auden (1907–1973) Anglo-American poet [Wystan Hugh Auden]

The Age of Anxiety: A Baroque Eclogue, Part 5 “The Masque” (1947)
    (Source)

Jean Kerr, in her play (1973) Finishing Touches, Act 1, the character Jeff paraphrases the above:

Remember what Auden says. Man is the only animal that learns by being hypocritical. He pretends to be polite and then, eventually, he becomes polite.

The idea of habit becoming character, or practicing one's way to (usually) virtue, can be found in numerous variations. See also Aristotle (c. 325 BC) (and again, and again), Plutarch (c. AD 90), Hawthorne (1850), Reade (c. 1860), Clarke (c. 1860), Roosevelt (1913), Lewis (1952), Hoffer (1955), Eco (1988).