“Sometimes you say such smart things,” says a mama to her mentally impaired son, “I wonder how smart you would have been if I hadn’t dropped you.” Not sure if that line deserves a chuckle or a wince? Then you’re ready for “Thick,” a comedy that thrives on keeping its audience off-balance. On paper, Rick Bland’s play might sound like the latest gimmick in the New York Fringe: Three actors play a dozen roles in the twisted tale of Rudolph (Bland), a “thick” (read: mildly retarded) man attempting to cope with his family’s collapse. Thankfully, the work itself eclipses the high concept. There may be only three actors, but with material this good, they create an entire heartbreaking world.

“Thick” has better jokes than a year of network sitcoms; hardly surprising, since Bland wrote off-Broadway gems like “The Complete Works of William Shakespeare (abridged).” But for all its wit, the play’s true theme is sadness. Every scene reflects some new burden, from the trauma of shoe shopping to the grief of accidental death. Having actors in multiple roles makes their individual sadness seem universal.

Laughs (and eventually tears) erupt as characters prove more or less able to deal with the indignity of living. On either end of the spectrum are the major supporting characters – Rudolph’s unstable Mama (Ross Mullan) and his willfully competent sister Louise (Tamara Bick). Narrating their lives from his man-child perspective, Rudy never understands the full implications of, say, Mama’s “non-alcoholic gin” or Louise’s “special boyfriends.” We, however, can see what he’s missing.

Mr. Bland’s elegant writing trusts us to feel the sting in Rudy’s naive observations. The actors trust us, too. Between them, Mr. Mullan and Ms. Blick fill every role but Rudy, but in none of them succumb to camp. Even the most peripheral characters – the two-line shoe salesman or the schoolyard bully – get played with a low-key reserve that reads as honesty. Quips and disclosures are never delivered with “actorly” intonations that announce when it’s time to feel something. Instead, bombshells drop casually, which gives them much more force.

Mr. Mullan deserves particular praise for making Mama a sympathetic wreck. He never resorts to a grotesque parody of femininity; he just slips on his horn-rimmed glasses, frets with his hands, and molds a dignified portrait of a woman whose addictions overcome her best intentions.

But what about Rudy? Mr. Bland gives himself the most difficult role, since Rudolph never leaves the stage and has to seem both simple and knowing. In a delicate performance, he makes Rudy far more than a simpleton. Though he keeps the same sing-song intonation throughout, he shades Rudy’s voice with growing emotion. We can see harsh truths begin to pierce sunny demeanor.

One plot strand in particular shows a brilliant collision between writing and acting. Throughout the play, Rudolph tries to grasp the meaning of the word “complicated.” His incomprehension reminds us he’s a symbol for the innocent happiness most adults lose. But his life just keeps getting more complicated, and his slow awareness of his messy life is undeniably moving. No one, it seems, is allowed to stay simple forever.

Despite its impact, Rudy’s symbolic function raises disturbing questions. What are we doing to ourselves when we allow the mentally impaired to represent our own lost innocence? Turning disabilities into symbols dehumanizes actually disabled people. It’s a symptom of modernity that we feel progress has robbed us of a less complicated past, and plays like “Thick” prove that artists can forcefully represent our longing for innocent days. We should al ways consider the consequences, however, of deciding that someone is enough of an outsider to represent the person we dream we used to be.

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In less heady news, the Fringe is also offering “Silence!” a musical adaptation of the Oscar-winning horror flick “Silence of the Lambs.” The show’s more fun than it has any right to be. Everything in the film – from Jodie Foster’s mealy-mouthed accent as FBI agent Clarice Starling to Anthony Hopkins’s slithering turn as Hannibal Lecter – is cleverly exploited. Best of all, some great musical numbers gleefully stray from the movie’s gloomy tone.

The less the show mimics its source, in fact, the better it is. As Lecter, Paul Kandel’s impression of Sir Anthony is rather flat. More triumphant are the supporting cast members, who spin single lines of dialogue into showstopping tunes. Dierdre Goodwin, for instance, almost steals the entire Fringe with her diva’s lament about playing Starling’s underwritten best friend. “Silence!” has plenty of those delightful surprises.

“Thick” will be performed again August 27, 28 & 29; “Silence! The Musical” on August 25 & 28 (212-279-4488).