Santino Fontana, left, and Tony Shalhoub (Joan Marcus/AP)

NEW YORK–As “Act One” would have it, no love is quite so intense and tempestuous as that between a playwright and his play. In fact, in the endearing new stage adaptation of Moss Hart’s memoir–long a theater world bible–any other affection revealed over the course of two and three-quarter hours in the Vivian Beaumont Theater is a pallid affair compared to the fervor of the dramatist savoring and agonizing over the reactions to his first Broadway play.

James Lapine, Stephen Sondheim’s collaborator on “Into the Woods” and “Sunday in the Park With George,” here directs his own adaptation of Hart’s account of partnering with George S. Kaufman on the 1930 comedy, “Once in a Lifetime.” And though Lapine’s overlong script could use some editing, and some of the myriad supporting performances feel under-realized, his “Act One,” which opened Thursday night, remains a warm and stimulating summation of the romance of the theater and the satisfactions of pleasing an audience.

If the prospect of following the painstaking progress to opening night for a hit play doesn’t intrigue you, then maybe “Act One” isn’t your show. Then again, the production offers other pleasures, most especially in the commanding turns of Santino Fontana and Tony Shalhoub. They play Hart-the-narrator at early and late stages of his career as a playwright (“You Can’t Take it With You” with Kaufman)  and director (“My Fair Lady”). For the scenes that are by far the evening’s best–those between the younger Hart and the more experienced Kaufman, as they fix Hart’s early drafts of “Once in a Lifetime”–Shalhoub plays Kaufman.  Best known for TV’s “Monk,” he does a superb job of conjuring a Kaufman whose tics and ill humor seem to feed both his legend, and his craft.

It’s through Fontana’s eyes as the awestruck Hart, born into a poor immigrant family in the Bronx, that we glimpse the neurotic Kaufman, who’s been persuaded by his wife Beatrice (Andrea Martin) that the younger man’s Hollywood satire has potential greatness in it. The story of their gradual meeting of minds over “Once in a Lifetime,” with Hart having to acclimate to Kaufman’s irascibility as they take the show through disappointing tryouts in Atlantic City and Brooklyn, consumes this evening’s second act. (The irony of “Act One” is that its peppy Act Two has it all over its more laborious Act One, concerned with Hart’s early exposures to show business.) The respect Shalhoub’s Kaufman develops for Fontana’s Hart, as the younger playwright digs in his heels, determined to work out all the kinks in the comedy, forms a deeply touching, emotional core for the show. And when at last in a curtain speech at the Broadway opening of “Lifetime,” Kaufman graciously pays tribute to his new writing partner, your heart can’t help but be full.

Set designer Beowulf Boritt puts the physical worlds of “Act One”–the Hart family’s tenement flat; the Kaufmans’ Manhattan townhouse; the theaters in which “Once in a Lifetime” finds its legs–on a huge turntable that seems to spin all night. Even if at some junctures “Act One” has you momentarily contemplating the benefits of jumping off, by evening’s end you’re happy to have invested the time in this rewarding Broadway merry-go-round.

Act One, based on Moss Hart’s autobiography, written and directed by James Lapine. Sets, Beowulf Boritt; costumes, Jane Greenwood; lighting, Ken Billington; sound, Dan Moses Schreier; original music, Louis Rosen. With Chuck Cooper, Mimi Lieber, Will LeBow. About 2 hours 45 minutes. Tickets, $127-$137. At Vivian Beaumont Theatre, 150 W. 65th St., New York. Visit or call 212-239-6200.