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Interviewing people about a figure as towering as composer Stephen Sondheim (and truth be told, in the theatre, none alive towers quite as high), one finds the superlatives are quickly spent. Brilliant, genius, national treasure — the words come up again and again. Such encomiums are regularly thrown around by stage artists, who tend to pour on the syrup when discussing colleagues on the record. But when the subject is Sondheim, who turned 80 on March 22, the terms are surrendered not with forced enthusiasm, but matter-of-factly, as if the speakers instinctively know they are his due and maybe even a little bit wanting.

From West Side Story in 1957 to Passion in 1995 and beyond, arguably no composer or lyricist working during the latter half of the 20th century has approached his contribution to musical theatre. Since his string of 1970s masterworks (Company, Follies, A Little Night Music, Pacific Overtures, Sweeney Todd), young writers can roughly be divided into those who have followed his artistic example of challenging topics, psychologically complex characters, nimbly intellectual lyrics and intricate, dissonant melody, and those who have strenuously strived to escape his influence. No single playwright or director has had as comparable an impact; among actors, perhaps the long shadow of Marlon Brando is a parallel.

"Everyone's influenced by him, even if they claim they're not," says Lin-Manuel Miranda, composer of In the Heights and a translator on the recent bilingual revival of West Side Story. "His shows have staked out such vast thematic terrain — murderous barbers, motherly witches, presidential assassins — that he's forever changed what a musical can encompass. In pursuing his craft, he's set the rest of us free."

Sondheim on Sondheim, a new revue of Sondheim songs currently playing on Broadway at Studio 54, reminds us of that craft. The show stands apart from previous revues (and there have been many) in that the man ("an ordinary New Yorker...with an extraordinary talent," the press materials state with a straight face) not only contributes two-dozen tunes but also his filmed thoughts and confessions about the tunes, how he came to write them and why. It's "Behind the Music," but on stage. The idea for the piece came from frequent Sondheim collaborator James Lapine, and the cast includes Barbara Cook, Vanessa Williams, Tom Wopat and Norm Lewis.

If past interpreters of Sondheim's music are any yardstick, those eight actors are currently laboring more than ever before — and are grateful for the strain.

"I worked harder than I'd ever worked to unlock a piece of music," says actor Michael Cerveris of his first professional encounter with Sondheim, in a Kennedy Center staging of Passion. With a couple hailed Broadway turns under his belt — Assassins, in which he won a Tony Award for his John Wilkes Booth, and the most recent revival of Sweeney Todd — he seems to have gotten the hang of it. Now other music is hard. During a recent workshop of another composer's musical, "I had the hardest time just singing simply in the key of C," says Cerveris. "Sondheim sort of ruined me for any other kind of music."

Patti LuPone, who starred opposite Cerveris' Sweeney as Mrs. Lovett, still thinks she hasn't sung the score to Sweeney Todd sufficiently well to suit her standards. "I'm conscious of what the notes should be," she says, laughing, "and I'm conscious of the notes that come out of my mouth. He's very complicated. The desire is to get it right, to be accurate in the lyric and accurate in the melody. It's no joke."

In contrast, Mandy Patinkin, who co-starred with LuPone in Evita before he created the role of post-impressionist painter George Seurat in Sondheim's Pulitzer Prize–winning Sunday in the Park with George — an experience that actor calls "the absolute seminal moment of my entire professional career" — perversely calls Sondheim's music "the easiest stuff to sing." And Patinkin has filled up concerts and albums with the writer's compositions. Patinkin delivers a Sondheim superlative that trumps the typical compliments. "[He's a] great professor of life," he reasons. "His students are all of us, both the performers and the audience. I think Stephen and Shakespeare are the two greatest teachers of the human condition that I've ever experienced."

Patinkin's Sunday co-star Bernadette Peters says something not dissimilar: "I'm constantly learning about life from his music and lyrics." And Barbara Cook furnishes a further echo: "I don't think anyone's written about the human experience with the depth that Stephen does. A lot of it is different, sometimes very difficult. But it's rewarding."

Barbra Streisand, who sang Sondheim compositions on both her "The Broadway Album" and "Back to Broadway," as well as many times in concert, expresses a concordant idea in two words: "Simply complex. His music gives me so much to work with as an actress...the intricacy of the lyrics...the layers of meaning. He always finds original ways to say things. Yet every word, every note, feels inevitable. He captures the truth in the most poetic way." Can one man, even one very accomplished man enjoying a big birthday, stand so much praise? Cook, who has known Sondheim since the late 1950s, thinks so. "In the last few years, I think, he is more able to accept his celebrity than before. That's why he's allowed this to happen. Still, he says he finds all this embarrassing."