“Those roles which, being neither those of Hero nor Heroine, Confidante nor Villain, but which were nonetheless essential to bring about the Recognition or the dénouement, were called the Fifth Business in drama and opera companies organized according to the old style; the player who acted these parts was often referred to as Fifth Business.”
—Tho. Overskou, Den Danske Skueplads (sic) quoted in Robertson Davies, The Deptford Trilogy: Fifth Business, (New York: Penguin Books, 1983). p. 3. [NOTE: According to the Wikipedia entry on this novel, this quote was fabricated by Robertson Davies.]