Emanuel Schikaneder wrote the libretto to The Magic Flute, K. 620. He also played the original Papageno (the birdwatcher). In the opera, Papageno sings with his magic chimes, and when he would pretend to strike the chimes, Mozart would play the chords on a keyboard glockenspiel offstage. During one performance, Mozart suddenly decided to play a short arpeggio on the glockenspiel where one wasn't written. Schikaneder figured that when the part would be repeated, Mozart would play the arpeggio again. However, when he went to strike the chimes, no noise came out. Mozart writes,
This time [Schikaneder] stopped and refused to go on. I guessed what he was thinking and again played a chord. He then struck the glockenspiel and said, "Shut up!" Whereupon everyone laughed. I am inclined to think that this joke taught many of the audience for the first time that Papageno does not play the instrument himself.Another time, the theatre in Prague was practicing the premier of Don Giovanni, K. 527, with Mozart conducting. One singer, Teresa Bondini, was supposed to scream at one point in the opera. Mozart didn't think the shriek was realistic enough. So, while the orchestra played and the singers rehearsed the scene, he snuck around backstage. When it came time for Bondini to scream, Mozart reached out and grabbed her. The frightened Bondini let out a good, loud scream. Mozart, satisfied, told her, "Right! That's the way to shriek."