Directing may be 90 percent casting, as the adage goes, but the ultimate effectiveness of The Light in the Piazza comes from an extraordinarily savvy - and daring - choice of source material. Elizabeth Spencer's novella (originally published in The New Yorker) was readily adapted as a 1962 film starring Olivia de Havilland. Making it the basis for a work of musical theater is an entirely different matter. The genre - especially in its current vogue - tends to favor a style of extroverted display that seems ill-suited to the fragile emotions of Spencer's touching story. Yet the show's creators, Adam Guettel and Craig Lucas, collaborate with a finely tuned sympathy. Their risk-taking attitude not only flouts the genre's conventions but turns this unusual narrative to aesthetic advantage. Here's a story about first love and heartbreak which, amazingly, we actually don't feel we've seen endless times before. As a musical, The Light in the Piazza latches onto this disarming quality to draw us into the world of its characters with an often-enchanting freshness. It's a bare-bones plot (warning: I'm about to spoil the suspense a bit) with lots of room for nuance: In 1953, Americans Margaret Johnson and her daughter, Clara, are spending a summer sojourn in postwar Florence. They chance upon an attractive and determined young Italian, Fabrizio Naccarelli. He and Clara fall in love with each other, but Margaret tries to nip their relationship in the bud, despite lively encouragement by Fabrizio's family. Her motive is to protect her daughter, whose emotional and mental development were arrested by a childhood accident, making her "too young for her age." Eventually Margaret comes to realize that her sheltering instinct is holding Clara back from life in all its fluctuations. There's a lengthy trail of production history leading up to the show's current incarnation, which opened Tuesday night at the Paramount (it runs there through April 29 as part of the current 25-city tour). Guettel began work on The Light in the Piazza in 1999 as both composer and lyricist. The "anxiety of influence" must get pretty personal when you're the grandson and son, respectively, of Richard and Mary Rodgers, but Guettel's ambition seems only spurred on by his heritage. Reportedly, his aim was nothing less than "to make the audience feel like they're in love." I missed the musical when it was first presented by Intiman Theatre in June 2003, but by all accounts its evolution since then has been significant. Playwright Craig Lucas, who came on board to write the book, also directed the original version. (Lucas and Guettel, incidentally, both make a part-time home in Seattle and are closely linked to Intiman.) In the next cycle of revampings for Chicago and New York, Intiman's artistic director, Bart Sher, took over as director. The more expansively designed Light now on display is the one staged at Lincoln Center in 2005-06. It picked up six Tony Awards and was has also been broadcast on PBS. It's not surprising that three of those Tony wins were for the design team (Michael Yeargan's sets, Catherine Zuber's costumes, and Christopher Akerlind's lighting). This Light is indeed visually impressive but elegant and economical. Seamless set changes easily shift among the titular Piazza della Signoria, the Uffizi, the interior of the Duomo, a bourgeois parlor for Fabrizio's family home, the Johnsons' hotel room, the Roman Forum (for a brief visit), and even the sterile mansion back in America from which Margaret is escaping. Akerlind's palette is imaginative: Sun-kissed cafes morph into church gloom and scary, De Chirico-esque night alleys; in a tour de force, the piazza opens up with a wash of light that becomes claustrophobic instead of liberating. The costumes play sexy Italian stylishness off against Clare's brightly childlike patterns and Margaret's straight-laced watchfulness. Guettel's score also won the Tony in 2005. His music isn't immediately loveable, working on subtler levels. And its sophistication is admirable while at the same time one of Light's weak spots - much like the inconclusive ending, which has a dramaturgical logic but still leaves you feeling not entirely satisfied. Much of the discussion around Guettel's music has to do with the lack of big-belter numbers and obvious, four-square rhythms. Instead, his strategy is one of understatement, leaving you waiting for climaxes that are rarely allowed to unfold. Guettel's lyrical touch is deft but often plays out as a haze of fragments - quite fitting, in fact, since much of the inner action is about suppressing memories, keeping emotions from blossoming. Ghostly harp glissandos and shimmery strings are Light's sonic world, along with angsty Sondheimian rustlings against oblique harmonies. The trouble is that, for all its nuances, the score can give an overall impression of bland, undifferentiated commentary. In some ways, it's the victim of Guettel's clear intelligence: The apparently saccharine strains of the Overture, for example, suggest a kitschy soundtrack from the '50s, as if meant to represent some fable of love Margaret has internalized. Guettel's lyrics can also come across as imprecise, though this may also be part of a strategy of suggestiveness. And when he nails it - as in "Dividing Day," Margaret's reflection on the true nature of her relationship with her husband - Guettel reaches a level of heartrending eloquence that easily compares with Sondheim. Guettel's songs aren't easily extractable from the show (one reason the Original Cast Recording gives such a pale reflection of Light's quality). All the more reason to admire Craig Lucas's brilliant construction in setting up their contexts. His excellent pacing of the exposition allows you to enter into Fabrizio and Clara's point of view entirely, with all the expectation of first love. Structural parallels between the two acts make room for poignant echoes. The use of comic relief in portraying the drama-inclined Nuccarelli family is fresh and unforced, while speeches directed across the fourth wall to the audience are cleverly orchestrated. Bart Sher's direction is beautifully calibrated, directing our attention as a painter uses composition to the emotionally resonant focus. Even a shared smoke between Margaret and Fabrizio's father becomes a cigarette ballet. Christine Andreas is worth the price of admission for her Margaret. It's a funny, wrenching, deeply felt portrayal, taking us through the arc from protective mother to someone who lets go and realizes the book on her own happiness hasn't been closed. Katie Rose Clarke's singing might be limited in expressive color, but her empathy for Clara's innocence and vulnerability is one of the production's most profoundly touching assets. As the zestful Fabrizio, the honey-toned David Burnham plays up the hammy theatricality his whole family is prey to. Dan Akroyd look-alike David Ledingham is a commanding paterfamilias as Fabrizio's father, while Diana Dimarzio contributes one of the cast's most sumptuous voices as his mother. Almost stealing the show in the Nuccarelli family scenes is Jonathan Hammond as clownish brother Giuseppe; Wendi Bergamini is his sharp-tongued wife. The couple's constant bickering, with its hints of La Bohème, is one of the show's many in-joke references to opera and musical theater.