One difference between poetry and lyrics is that lyrics sort of fade into the background. They fade on the page and live on the stage when set to music.

Sondheim draws a practical distinction between literary poetry and theatrical lyric-writing. A poem is designed to be encountered as language on the page, where its verbal texture and self-sufficiency are foregrounded. Lyrics, by contrast, are written to collaborate with music, performance, and dramatic situation; they are meant to be “heard” and embodied, not merely read. When successful, a lyric can seem deceptively plain in print because its full force emerges only when rhythm, melody, and an actor’s intention animate it. The remark also implies an ethic of craft: the lyricist serves the song and the stage, allowing words to recede so the total theatrical moment can come forward.