The composers were in charge of writing the music and then actually playing the rhythm section for the operetta. They had to rehearse and learn how to play the instruments and read music so that when the show came, they would be ready. My job in the opera was a composer.
Composers write music.
My job entailed making beautiful music for every scene. I liked my job because I like writing music and composing. I was a composer. Being a composer entailed writing the notes for the songs. It also entailed making sound effects and playing the music during the show.