That line is set to a musical phrase that coils in on itself for the first clause and leaps in joy for the second. If no one has ever exploited the thousand colors and astonishing coincidences of English to greater effect than Sondheim, that’s in part because no truly great lyricist, save possibly Cole Porter and Frank Loesser, has been his own composer.

But even more than for those showmen, Sondheim twins and twines the two elements like DNA. Sometimes that means letting the music support the words, intensifying them, as in the song “Color and Light” from “Sunday in the Park.” While the pointillist painter Seurat dabs dots of pigment on his canvas, the lyric (“Red red red red/red red orange”) underlines the action and is, in turn, underlined by the music’s bristly staccato.

Other times the music sends a coded secondary message, contradicting the lyric. Take “In Buddy’s Eyes,” from “Follies,” in which the accompaniment turns reedy and sour whenever the despondent wife, who sings what she thinks is a tribute to her husband, starts lying about her “ducky” life. (Jonathan Tunick has orchestrated many of Sondheim’s great works, with the daring and inerrancy of a sherpa.)

And sometimes, most thrillingly, Sondheim will double-team the drama, pitting both music and lyrics against the story. In the song “Pretty Women” from “Sweeney,” as the voluptuous tune and ethereal lyrics (“dancing” and “glancing” rhyme with “how they make a man sing”) pulse toward what feels like erotic release, the vengeful barber is stropping the blade that will soon kiss his customer’s neck. The gasp is in the gap between what we hear and what we know.

Conflict like that is the essence of drama, which is why musicals too often seem thin when they try to approach the density of plays. Their emotional states are usually monaural, offering only one channel of perception at a time. The cowboy is happy, so he sings “Oh, What a Beautiful Mornin’.” The couple are either in love and say so, or in love but pretending not to be. The music may be delicious, the lyrics clever, but the situation is flat and generally inert; the songs are the release from the story instead of being the story itself.