Theatrical bitterness, Broadway’s obsession with movie stars, and affairs of the heart get a well-deserved ribbing in Theresa Rebeck’s new sweet and salty satire-cum-love letter to the theatre, The Understudy, which due to popular demand, has been extended through mid-January at the Roundabout.

Harry (Justin Kirk) is a frustrated theatre actor who is understudying Jake (Mark-Paul Gosselaar), a studly action movie star seeking a bit of legitimacy by performing in a newly discovered Kafka dramatic masterpiece on Broadway. At the most inefficient understudy rehearsal ever, the frazzled stage manager Roxanne (Julie White) attempts to sooth a Hollywood ego while keeping her own emotions in check – several years before, Harry had jilted her two weeks prior to their wedding.

From the opening moment in which Harry somersaults onstage with a prop gun and launches into a hilarious diatribe against the Hollywood machine and Jake’s recent hit movie, you know you’re in good hands. Justin Kirk, an award-winning theatre actor who is nonetheless better known through his television performances in “Angels in America” and the hit Showtime series “Weeds,” brings a full arsenal of tics and quirks to the embittered Harry, who never got the break he feels he deserves and is indignant about covering a "talent-free" B-level movie star.

Meanwhile, Jake is not exactly pleased that his understudy is a twitchy little schmuck. Mark-Paul Gosselaar (TV’s “Saved by the Bell” and “NYPD Blue”) is making an assured and confident stage debut in The Understudy. Dressed in a tight black t-shirt and designer jeans (costumes by Tom Broecker) he perfectly captures Jake’s Hollywood bravado while exuding the character’s genuine affection for the preposterous Kafka drama in which he is performing. In a scene which alone is worth the price of admission, star and understudy warm up to each other as Gosselaar teaches Kirk proper gun-toting technique while intoning the signature line (“Get in the truck!”) from his latest action blockbuster.

Meanwhile, Julie White, a Tony Award-winner for The Little Dog Laughed who also single-handedly made The Transformers film franchise bearable for grown-ups, does yeoman’s work here as the frazzled Roxanne who pushed aside her own dreams of performing in favor of a steadier career in stage management. Whether she’s rolling her eyes while frantically trying to hold the rehearsal together, or exposing the cracks in her brittle demeanor, White handles the plays’ farcical and emotional elements with aplomb and reminds the audience why she is a theatrical treasure.

Director Scott Ellis keeps his stellar trio of actors firing on all cylinders, nailing every laugh in Rebeck’s script. The designs are also first-rate, including Alexander Dodge’s imposing, fluid sets for the play-within-the-play and Kenneth Posner’s gorgeously moody Kafka-esque lighting. The design elements also provide some of the biggest laughs of the show as an unseen tech supervisor incorrectly changes the scenery between bong hits.

In the play’s finale, White, Kirk and Gosselaar perform a giddy soft-shoe routine that concludes the Kafka drama. Likewise, the audience dances out of the Laura Pels Theatre on a buoyant high that only a great evening at the theatre can provide.

The performance schedule for The Understudy is Tuesday through Saturday at 7:30pm, with Wednesday, Saturday, and Sunday matinees at 2pm. Tickets are available by calling Roundabout Ticket Services at 212.719.1300, online at www.roundabouttheatre.org, or at the Laura Pels Theatre at the Harold and Miriam Steinberg Center for Theatre box office at 111 West 46th Street. Ticket prices range from $70 - $80.