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It's the understudy, the stage manager and the actor with second billing that finally get the spotlight, in Theresa Rebeck's latest work — aptly titled The Understudy. This backstage tale about the underbelly of showbiz stars theatre pros Justin Kirk and Tony Award winner Julie White along with New York theatre newcomer Mark-Paul Gosselaar. Under the direction of Scott Ellis, White attempts to hold court in this three-person comedy, as she rehearses Kirk (the understudy) and Gosselaar (the supporting lead) in a rediscovered Franz Kafka play.

First seen at the Williamstown Theatre Festival last year (with Bradley Cooper, Reg Rogers and Kristen Johnston), The Understudy, now at the Roundabout's Laura Pels Theatre, is centered on a play within a play. For Gosselaar, consider it a New York theatre debut within a debut.

Best known for creating the character of Zack Morris on the '90s TV show "Saved by the Bell," Gosselaar plays Jake, the supporting lead and standby in the play within Rebeck's actual play.

Gosselaar's co-star, Justin Kirk, knows quite well of the benefits stage acting has on a well-rounded TV career. Widely known for his role of Andy Botwin on Showtime's "Weeds," Kirk returns to the New York theatre scene after a seven-year absence and plays the character of Harry, Gosselaar's understudy.

"This show is different than anything I've done in theatre," says Kirk. "Theresa [Rebeck] writes great parts for actors. There's something sad and funny about this particular character and I was excited about doing it."

As for working with Gosselaar? "He's a total theatre newbie — but he's also been the lead on a television show for the past 20 years, so he's not," Kirk says. "[Gosselaar] knows his way around a part."

Mark-Paul Gosselaar may be new to theatre audiences, but for television, not so much. He's been a longstanding presence on early morning TV, thanks to his role on "Saved by the Bell" (which still plays in syndication), and more recently his work on "NYPD Blue" and TNT's "Raising the Bar." He says he welcomes fans of his television work into the theatre. "I don't shy away from contact with the fans or people who enjoy watching me on other things. As a performer I thrive off the energy of the audience. It helps the production that we have an energetic audience that's willing to laugh at our jokes and follow us along in the journey we're trying to tell."

Gosselaar says he's looking forward to this new chapter in his professional life. "I'm looking to make a relationship here in New York with the theatre community," he says. "I would come back here in a heartbeat."