“Where you going?” a man asks the woman leaving his bed one morning — possibly expecting her to say, “to the bathroom.”

Instead she says, “Barcelona.”

Or, rather, she sings it, because the joke as well as the character insight — she’s a stewardess — are part of a song that became its own three-act mini-drama in the 1970 musical “Company.”

Act One: Bobby, the man, tries to get April, the stewardess, to come back to bed but fails.

Act Two: As April puts on her uniform, Bobby rhapsodizes about her being a very special girl — “and not because you’re bright.” (He quickly corrects himself: “not just because you’re bright.”) Then, on a ringing high note, he calls her June.

Act Three: When she accedes to his relentless importuning, he is instantly horrified. “Oh, God,” he sings, having achieved the companionship he never wanted. Blackout.

What just happened? In the three minutes, 93 bars and 181 words that make up the song “Barcelona” — one of 15 or so in “Company” and more than 750 in the catalog of Stephen Sondheim — theatergoers get a complete narrative, within the larger one of the show, that deepens our understanding of Bobby, bachelorhood and the push-pull of otherness. The director (in the original production, Harold Prince) gets something too: a rich scene to stage, the actors, a palpable conflict to play and the subtext to inform it.

And all this is done in classical A-B-A form, to a sweetly lazy tune befitting the morning-after setting, with apt but gentle rhymes (“going”/“Boeing”) and punchlines that are not just punches. They help you sympathize a little more with Bobby, even if you like him a little less.

No one has so consistently and so widely across tonalities and topics worked that “Barcelona” effect like Sondheim. Which is why I’ve come to feel that, for all his accolades — eight Tony Awards, five Oliviers, an Oscar, a Pulitzer Prize and a Presidential Medal of Freedom — he is still, turning 90, underacknowledged.

To say that he has been revered as a brilliant trickster lyricist for at least 60 years, and for 50 as a composer of singular breadth and passion, minimizes his achievement. Not that I don’t share in those judgments; I’m as thrilled by his work today as I was when first introduced to it by the original cast recording of “Company,” which I transcribed word by word from my parents’ cassette tape because I didn’t know how else to absorb it fully.

But listening again to the 15 major stage works for which he has served as both composer and lyricist, I find myself thinking not of Sondheim the word man or of Sondheim the music man but of Sondheim the dramatist. Having long taken for granted that he is the greatest composer-lyricist the United States has produced, we can perhaps now notice that he is also an artist to place in the line of America’s foundational 20th-century playwrights. In years to come, critics will have trouble understanding how our time put him in one basket but put Tennessee Williams, Arthur Miller, August Wilson and Edward Albee in another.

Well, they’ll understand this part of it: Musical theater is a much more collaborative field than nonmusical theater; Sondheim never wrote the books for his own projects, rarely even initiating them. (Exceptions: “Sweeney Todd,” “Passion,” “Road Show.”) But one of the things that the people who did write those books must have learned to accept, and in some cases treasure, is the way he cannibalized their work until not much was left of it but the bones. Sometimes he even ate the bones; “Barcelona” is the entire scene, no dialogue needed.

So while the stories of all 15 shows were collaborations, by the time they got musicalized, the drama was mostly Sondheim’s. That’s especially obvious in the great works built on middling books. In “Company” and “Merrily We Roll Along” (both by George Furth) and “Follies” (by James Goldman), the spoken parts are almost always where the action isn’t; you sit through them to get to the songs.

The proof is everywhere. When directors looking for challenges (or engaging in rescue fantasies) overhaul the Sondheim oeuvre, it’s not his contribution they try to reinvent. Take the new version of “Company”, set to open on Broadway this month, in which director Marianne Elliott (with Sondheim’s encouragement — he’s the least precious auteur ever) keeps the score pretty much as it is but flips prototypical toxic bachelor Bobby to a biological-clock-watching woman named Bobbie. Or “Merrily,” that great white whale of musical theater aspiration, with a hundred Ahabs bent on landing its jaw-dropping score and unworkable book, even if it means building a new whale in the process.

If I sometimes wish that Sondheim, in his loyalty, had not lavished so much of his gift on material he had to elevate, it’s also the case that those works allow for elevation. Still, the dramatizing effect of Sondheim’s scores is more profound when both he and his collaborators are at their best.

It’s a matter of taste, but I would include in the best-book category “Sweeney” and “A Little Night Music” (by Hugh Wheeler) and “Passion” and “Sunday in the Park With George” (by James Lapine). In these musicals and a few others, the raw material allowed Sondheim to create sequences in which neither music nor lyrics prevail but their interplay produces the heat of lived emotion.

How does it happen? How does he get so often to the place where real drama happens, when even the greatest of his predecessors could get there only occasionally?

Part of it is technique, both verbal and musical. It’s easier to talk about the verbal kind because Sondheim’s lyrics represent such a quantum improvement over the vagueness and inanity of almost everything that came before. As befits a man who for a time wrote British-style cryptics, his words serve many purposes: delight, emphasis and subversion among them.

His use of rhyme is only the most obvious aspect of this. Whether quiet (like that “going”/“Boeing” matchup from “Company”) or showy (“personable”/“coercin’ a bull,” from the same show), the pairings almost without exception scan perfectly and, beyond that, highlight meaningful connections between sounds instead of an over-clever Lorenz Hart-like coupling that amounts to a shotgun wedding. It’s telling that even Sondheim’s non-rhymes rhyme, in the sense that they link different aspects of the same word. In a song called “Isn’t He Something!” from “Road Show,” when the mother of two sons exults in the disreputable one’s charisma, the lyric reflects the deviousness of language and thus of character: “If he had the slightest sense of shame/it would be a shame.”

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 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.

Rodgers and Hammerstein transcended that template but only so far. Sondheim’s study of their innovations (Hammerstein was his mentor) led him to workarounds that multiply the information available in any one moment of his shows. Sometimes it seems that even his subtext has subtext, putting down roots in the deepest murk of human complexity. It’s that complexity that has made so many songwriters of the ensuing generations into Sondheim slaves, seeking to re-create in their own voices the many aspects of his. Some get close, but there is a unique feeling of compression and perfectionism in a Sondheim score: Every musical gesture and every lyric does at least double duty. Exposition is character. Melodies are psychology. Rhymes are revelations.

Of a stale marriage: “Mildew/Will do/Harm.” And something even bigger happens when he is given space by his book writers — or, I suspect, when his own deepest engagement in the material causes him to commandeer that space.

I’m talking about extended musical scenes like the opening sequence of “Sweeney,” with its screeching factory whistle, its busy ballad, its turbulent Thames-side mutterings, its comedy charm and yearning regret. At the end of it, Sweeney sings his only love song — the sinuous “My Friends” — to his razors.

Or the letter scenes in “Passion,” turning the question of romantic love this way and that like a fidget spinner.

Or the first act finale of “A Little Night Music,” in which a plot as intricate as a murder mystery gets puzzled out in a nearly seven-minute number called “A Weekend in the Country.” Wheeler’s interstitial dialogue functions like tendons as the song muscles on, expressing disagreement in laugh-out-loud rhymes (“depraved”/“engraved”) and excitement in an ear-worming tune, all of it tied up in an operetta bow with a thrilling Straussian climax. Such passages barely touch down to refuel. In their breadth and daring, they are akin to opera, except that the words are not a sequence of singable vowels but real thoughts that ripple with specificity. Expressing through conflict all manner of insight, they are thus less closely aligned with the grandeur of Puccini and Verdi than with Miller’s moral acuity, Williams’s poetry of human failure, Wilson’s outsider fury and Albee’s raised-eyebrow, zinger-filled existentialism. Through the shapes of melody they paint the lines of character. They do more than sing: They tell stories that move.

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