Theresa Rebeck, please know that we get it. We get that show business is a cruel, crazy and sometimes wonderful world, because we've heard it over and over and over again—usually from you. And The Understudy is a lesson in diminishing returns.

Rebeck's new comedy would be a better show instead of a merely entertaining one if the playwright (whose Our House this summer was so bad I almost quit the theater world on the spot) wasn't so damn lazy. Set during an understudy rehearsal for a Broadway production of a long-lost Franz Kafka play, understudy Harry (Justin Kirk) is learning the blocking from stage manager Roxanne (Julie White) and Jake (Mark-Paul Gosselaar), the Hollywood star Harry is understudying. Except Jake is also an understudy. Yes, in Rebeck's world, an action star is not only appearing on stage in a Kafka play, he has the additional strain of knowing both the show's roles inside and out.

I can't begin to explain how annoying this device is, especially for this show's target audience. Presumably, Rebeck understands that the people who come to see backstage comedies are theater savvy, yet she deliberately creates a wholly unbelievable premise. And her easy jabs at Jeremy Piven and movie stars on Broadway come across as highly disingenuous, given that Kirk and Gosselaar are better known for their television roles than for any extensive stage credits, and that the Roundabout is producing the show in the same season they lobbed their John Stamos-starring Bye Bye Birdie at audiences.

But if you can somehow surmount the implausibility of a movie star who earns $2 million a movie throwing himself into his role as an understudy with glee, there's a legitimately fun time to be had. Much of the evening's success can be attributed to Julie White, everyone's current favorite comedic leading lady. As Roxanne struggles to subsume her own acting ambitions in her new position as stage manager, keeping egos soothed and things running smoothly, White veers between hysterical and comforting. Her takedown of Kafka's sexism is a comedic highpoint in a show that too often settles for plateaus.

By now, White is expected to be fabulous, and Kirk, after several seasons on Weeds, transplants his smarmy, charming jerk routine to the stage successfully; the real surprise here is Gosselaar. Looking like a million bucks—thank you, costume designer Tom Broecker, for putting him in a tight T-shirt and tighter jeans—his comedic timing proves that his casting wasn’t some misguided attempt at filling the Roundabout’s stages this season with bygone pop culture icons. His Jake is surprisingly smart for an actor who screams “Get in the truck!” and earns two million dollars for it, even if his newfound devotion to the theater is doomed to end in heartbreak.

And though Rebeck frequently fails her story by taking the easy way out, her ending is a weird and wonderful moment between the three characters, all of whom brush aside their disappointment at the vagaries of show biz to dance with childish abandonment. The Understudy could have used more moments like its final one, because too often the show feels like its title: a competent substitute for the real deal.

Through Jan. 3, the Laura Pels Theatre, 111 W. 46th St. (between 6th & 7th Aves.), 212-719-1300; times vary, $70$80.