Both Tharp and Joel are quick to say that “Movin’ Out” is not a musical in the conventional sense. “We can’t come up with a name for what this is,” says Tharp. “This is not a book musical; we do not have book scenes.” Or dialogue. But the show has characters and a narrative line, expressed in dance, that Tharp has pieced together from Joel’s songs. “Movin’ Out” follows a group of working-class baby boomers from New York’s Long Island, who couple and uncouple, and endure the destructive forces of Vietnam and the counterculture. Joel’s songs–from the more innocent “Uptown Girl” to the darker “Angry Young Man” and “Goodnight Saigon”–are played by a live rock band, as Tharp’s crew dances the action in her famously eclectic style, from the sweet and balletic to the violently kinetic. “I always felt Billy could do a Broadway show, and a Broadway show was something that always interested me,” says Tharp. Two years ago she called him and proposed the idea. “I didn’t want to do something like this before,” said Joel, on the phone from his boat off Long Island. “People have sent me cornball, cliched scripts about ‘Piano Man’ or ‘Uptown Girl.’ But Twyla approached me about choreographing to the music, and I thought it was a natural.” Tharp had choreographed some of Joel’s music, and he went to see it. “I was really moved,” he says. “The dancers had become these people in the songs.”

It’s an intriguing concept, if tough to pull off. In the Chicago tryout, “Movin’ Out” was hammered by some critics. Tharp rolled up her sleeves and began “pruning,” especially the problematic first act. Friends such as director Mike Nichols offered suggestions. Joel himself had weighed in early, recommending musicians for the band, including a singer/piano player Joel saw in Vegas who’s his doppelganger in the show. “But Twyla made the ultimate decisions,” he says with a laugh. “If you stand in Twyla’s way, you die!”

Tharp’s choreography always lets her dancers’ personalities emerge, which is critical for a show like this where the characters don’t speak. Six of the principals have toured with Tharp’s company, and they’ve got her demanding technique nailed–you’re not likely to see such virtuosity on any other Broadway stage. Still, Tharp’s been bruised on Broadway before: she took a critical beating for her 1985 staging of “Singin’ in the Rain.” But the fate of “Movin’ Out” is more likely in the hands of fans than critics. To anyone who thinks Joel’s and Tharp’s fans couldn’t overlap, Twyla has this to say: “The guy on the street has always been my hero, just the way the guy on the street has always been Billy Joel’s hero.” Tharp at 61 is as determined as ever. She even says she’s likely to perform again herself. “I know only this–You never say no to anything.”