A musician cannot move others unless he too is moved. He must feel all the emotions that he hopes to arouse in his audience, for the revealing of his own humor will stimulate a like mood in the listener.

As quoted in Così? : Sexual Politics in Mozart's Operas (1991) by Charles C. Ford, p. 46

A musician cannot move others unless he too is moved. He must of necessity feel all of the affects that he hopes to arouse in his audience, for the revealing of his own humour will stimulate a like humour in the listener. … constantly varying the passions, he will barely quiet one before he rouses another. Above all, he must discharge this office in a piece which is highly expressive by nature, whether by him or someone else. In the latter case he must make certain that he assumes the emotion which the composer intended in writing it.