Brash Hollywood agent Diane harbors no illusions about the movie biz she loves, loathes and can't live without. And from her opening salvo about the naive matinee idol she's grooming for mega-stardom, actor Karson St. John fills the role to the brim -- and then some.

St. John is the best, though not the only, reason to catch Diversionary Theatre's new production of Douglas Carter Beane's "The Little Dog Laughed." She nails a role that, three years ago, the hyperkinetic Julie White electrified and seemed to own forever in New York. The estimable White won a Tony for her acid and hilarious portrayal of this wily manipulator, a woman whose machinations are so firmly rooted in real insight and tawdry truths about people that she's likable in spite of herself.

Diane has come east from L.A. with her main meal ticket, actor Mitchell Green. Among other functions, she'll be his date at an awards ceremony. Better her than his mother, she tells us. (He's innocent enough to think bringing his mother along would deflect any suspicion that this purported sex symbol is gay.)

Curvy and elegant in a form-fitting, spangled and ruched sapphire sheath, St. John delivers Beane's witty and energetic goods in an opening monologue and a later show-stopping riff on legal contracts escalating into infinity and beyond. First seen locally in another motor-mouth comic role at Ion, the actor owns the confident rhythms and big synthetic smile that tell you she's in charge of the evening -- and the relationships unfolding all around her.

After stints as a movie and TV writer, Beane emerged as a playwright with his breakthrough comedy, "As Bees in Honey Drown"; he also did the books for the Broadway and La Jolla Playhouse productions of "Xanadu" and the Globe's still-evolving "Dancing in the Dark." He structured "The Little Dog Laughed" as a series of spotlit interior riffs, delivered by characters side-by-side or alternating and intercut with more conventional encounters.

One unexpected delight of the Diversionary production is the speed and expertise with which director Robert Barry Fleming moves his cast from one mode to the other, never skipping a beat or missing a laugh in Beane's crackling, one-liner-strewn writing.

Diane is the keeper of the plot's central conceit: In Hollywood even now, a matinee idol can do anything he wants in the closet; he just has to shut up about it in public. He can even play a gay role and earn Brownie points for nobility, so long as the paying public thinks he's straight.

And that's another reason the agent and her charge are in New York. She enlists him to help her secure the movie rights to a play featuring two gay men. He'll be attached to the "property" as the star. One high point is their luncheon seduction of this famous playwright, previously burned by his work in Hollywood.

After giving "her word" that she'll maintain the integrity of the playwright's work on film, Diane confides, "A writer with final cut: I would rather give firearms to small children."

Beane works new permutations on the old movie-code hypocrisy by giving the star, Mitchell (Brian Mackey), and his new squeeze, the young rent-boy Alex (Bryan Bertone), denials and doubts about themselves. Add Diane's candor about being a lesbian who gets off on work, not sex, stir in Alex's party-loving "girlfriend" with her own confusion about what she wants out of sex and life, and the play has plenty of serious resonance.

But give those serious moments too much heft and the bubble bursts. One questionable choice at Diversionary has Mackey as the movie star going all Method-y during a particularly intense scene of self-discovery toward the end. Young Mackey is not quite mature and beefy enough for the part to begin with. When Mitchell's dilemma plays too real, we feel too much empathy for his dilemma and the satire curdles.

Mackey was first paired with Bertone in the more layered writing of Alan Bennett's "The History Boys" earlier this year at Cygnet. As Alex, Bertone has the great looks and convincing sincerity of a street-smart kid who might actually open to love with a messed-up movie idol.

Bertone is especially good in Alex's pretending-to-be-asleep monologue, ping-ponging from denial to final acceptance that he's gay. Beane's writing is riotous here as the kid waffles, stream-of-consciousness-style, from feeling "gay as a goose" to thinking he couldn't be: The minute anybody on screen "even looks as if they're going to sing, I'm embarrassed for everyone," he says.

Meanwhile, his terminally hip girlfriend Ellen (Kelly Iverson) deadpans lines like "We're 24; hope is dead." Iverson got off to a shaky start on opening night, but hit stride as the show moved her character toward Diane's engineered surprise ending.

Aside from the occasional tonal miscue, Fleming, with big assists from designers Chris Renda (lights), George Ye (sound) and Jennifer Brawn Gittings (costumes), here makes a strong directorial debut at Diversionary.

While the U.S. military considers changing its muddled "Don't Ask, Don't Tell" policy, St. John's savvy Diane with her elegant clothes, striking jewelry and shrewd wisecracks, knows better about Hollywood.

"You can love or do whatever you want," she tells her faltering client Mitchell, "All you have to do is shut up!"

"The Little Dog Laughed"

When: 7:30 p.m. Thursdays; 8 p.m. Fridays and Saturdays; 2 and 7 p.m. Sundays; through May 31

Where: Diversionary Theatre, 4545 Park Blvd., San Diego

Tickets: $29-$33

Phone: 619-220-0097

Web: www.diversionary.org