On Beethoven in Peanuts:
"If you don't read music and you can't identify the music in the strips, then you lose out on some of the meaning," said William Meredith, the director of the Ira F. Brilliant Center for Beethoven Studies at San Jose State University, who has studied hundreds of Beethoven-themed Peanuts strips.
When Schroeder pounded on his piano, his eyes clenched in a trance, the notes floating above his head were no random ink spots dropped into the key of G. Schulz carefully chose each snatch of music he drew and transcribed the notes from the score. More than an illustration, the music was a soundtrack to the strip, introducing the characters' state of emotion, prompting one of them to ask a question or punctuating an interaction.
Listening to Schroeder: Peanuts Scholars Find Messages in Cartoon's Scores (New York Times)