First thing: Forget the movie. Just forget it; it’ll only get in the way.

Forget how an inert Rock Hudson occupied the center of that 1956 film like the beefy ball of bland he was. Forget the perpetually bewigged Liz Taylor, forget James Dean’s squinty eyes and Boomhauer-esque mumblings. Forget how well director George Stevens evoked the heart-stopping, dun-colored vastness of Texas and forget—please—how the filmmakers set out to decry racism even as they were slapping brown “Mexican” makeup on Sal Mineo.

Put it all out of your mind, because this Giant—Signature Theatre’s hugely ambitious, well-built if overlong world premiere musical—goes back to the original source, Edna Ferber’s 1952 novel. What director Jonathan Butterell is after, here, is nothing less than a ground-up reimagining of Ferber’s Texas-sized multigenerational saga. And he pretty much achieves that the moment you clap eyes on Dane Laffrey’s stylishly sleek set: a wide, black, empty stage, a single raised platform and, suspended above the action, a thin rectangular bar of light to evoke the featureless horizon, cycling slowly through days of flat white heat, golden sunsets, and cold blue dawns.

Under that patch of merciless Texas sky, young cattleman Jordan “Bick” Benedict (Lewis Cleale) will bring Leslie (Betsy Morgan), his independent-minded Northern bride, to live. There, she will clash with Bick’s sister Luz (Judy Blazer), fend off creepy ranch-hand Jett (Ashley Robinson, having a ball), and raise two children. Decades will pass, and rancher Bick will find his way of life threatened by a son (Jordan Nichols) who has his own ideas on what to be and whom to love, and by the coming of the oil industry, which will force Bick to choose between his land and the wealth that may lie beneath it.

It’s worth noting here that a lot of people are paying a lot of attention to this production. This filly’s got pedigree: Butterell choreographed Broadway’s The Light in the Piazza and a highly acclaimed revival of Sondheim’s Assassins. Signature itself just won the 2009 Regional Theater Tony Award. And then there’s Michael John LaChiusa.

You might not know his name yet, but your theater-geek friends do. Ask them, and they’ll talk about his music—how his melodies have a way of doubling back on themselves to unearth something surprising, how his adventurous, muscular harmonies and counter-harmonies make unexpected circles and swoops. And they’ll talk about his lyrics, which tend to come at their subjects from oblique angles.

Unfortunately, that LaChiusa remains MIA for most of Giant’s lengthy first act, and doesn’t really show up until we’re a good hour-and-a-half into the show’s nearly four-hour running time.

As a result, too much of that first act bears a disquieting resemblance to musical theater parody, with all its attendant sudsiness and brio: “Did spring come to Texas?/Am I just goin’ loco?” sings a love-smitten Bick before launching into a list of some native desert flora. (He’ll get to listing the fauna in another song that comes just before the act break.) There’s an alarming amount of similar business, all of it way too on-the-nose, and it’s puzzling. You keep waiting for LaChiusa’s lyrical inventiveness to kick in, for a song’s imagery to catch your imagination, to surprise.

But here’s Bick on love: “I’m crazy like a buck catchin’ scent of a doe.” And here’s Jett, comparing women to cars: “Who gets your motor purrin’?/Who’s the only one/Who gets your juices stirrin’?”

It doesn’t take long for all those dropped g’s to start crunching under the actors’ feet—and that’s when it strikes you that what you’ve been watching seems merely dutiful, each song just another box to be ticked. Giant’s first act is filled with some of the most commercial, easily accessible, and least layered music LaChiusa’s written, as if the act of yoking his discursive impulses to such a straight-ahead yarn has broken his spirit.

But just then Katie Thompson saunters to the center of the stage as Vashti, the local gal whose attentions Bick has publicly spurned, and commences to sing the holy hell out of the plain but beautiful “He Wanted a Girl.” That moment cracks Giant wide open, as Thompson’s smoky voice deftly uncovers the layers of mood and meaning the production had been missing. And once she’s loosed them, they hang around, enriching what comes after. She’s pretty terrific.

By the second act, as LaChiusa starts weaving together the musical themes he introduced in the first, the characterizations get a much-needed chance to deepen. Cleale, who seemed a bit uneasy with all those unalloyed songs of love, gets more to chew on. Morgan’s Leslie lets loose on the flirty “Topsy Turvy”, and Robinson—whose performance as Jett starts out big and sleazy and just gets bigger and more joyfully sleazy with every number—brings a raw honky-tonk sexuality to the fore.

The third act can’t match the second’s note-perfect charm—the talky final scene between Bick and Leslie cries out for trimming—but it’s where LaChiusa gets room to experiment, as when he allows Latin rhythms and instrumentation to color and complicate a familiar reprise.

Given the running time, it’s only natural to ask if every one of the evening’s 29 songs has been thoroughly interrogated and forced to justify its presence. My sense is that they have not—especially early on, when characters break into song for reasons that never become sufficiently apparent.

But it’s tough to knock a show for self-indulgence when said indulgences include allotting so many crowd-pleasing numbers to performers like Katie Thompson and John Dossett. (Dossett, whose bluff, larger-than-life Uncle Bawley still seems shoehorned into the proceedings and isn’t yet pulling his full narrative weight, packs a baritone you feel in your chest.)

In place of the 1958 film’s good-hearted but ham-fisted take on the evils of prejudice, Butterell conducts a darker, more nuanced examination of race and class. Mexican ranch-hands and servants share the stage with the wealthy Anglos throughout the play, but the script doesn’t permit them a voice of their own until the final act, when we see many old barriers begin to crumble.

Which sets up a subtle, lovely moment near the midway point: Leslie stands amid a group of Texans as they belt out a jingoistic paean to their home state. Suddenly the party freezes, a spotlight isolates her, and the song she sings to herself—“Your Texas”—rebukes the life she’s made among these hard, cruel people. Far across the stage, director Butterell lets a softer, angled light fall across the face of a Mexican servant (Isabel Santiago), who, as if able to hear Leslie’s song, silently begins to weep.

The spotlight fades, the lights come up, and that connection between the two women vanishes as quickly as it appeared. But

the memory of it lingers over

the production like an extra, unspoken layer of meaning that offers something you never expected to find in Ferber’s now-familiar tale—an arresting, and welcome, surprise.