The theater critic sits in a folding chair marked “RESERVED” just to the right of center stage. In his lap: a notebook and a pen.

“A review almost exists before it’s written — that’s the painful thing,” he says.

I wonder what irony has placed me, a novice theater reviewer, in the front row directly across from an actor who is playing my part. I close my notebook feeling very much under observation.

But then, that’s the point of “Critical Mass,” playing through Saturday at the Whitney Humanities Center: to reflect back the experience, to make us see plainly the range of the human capacity to wrap our minds around things and then put them in their place.

“Criticism is the contextualization of our experience,” Cordelia Istel ’10, who narrates the show throughout, explains with authority. In a simple brown dress with hair drawn back in a ponytail, she shines as the candid but insecure writer / director / professor Deb Margolin’s younger self; she bridges the gap between audience and show by alternately focusing and blurring the performance with the act of critiquing the play.

This is a show of 10 actors, a set consisting of a folding chair, minimal costumes and a spotlight — a spotlight on emotion and response. When Istel interrupts the show to flick on the house lights, the actors crowd into the audience’s seats, perching on our laps and around feet.

They criticize the script. They expose the show’s flaws. They ask the questions and make the snide remarks that we as audience members may have been considering. The effect is disorienting as their jarring voices ring out from beside us; we are forced to ask whether the viewer belongs in a play that emphasizes the centrality of the viewer.

We cannot create our own perception of the show because Margolin’s actors do it for us. We are told what to love and what to hate before we have the chance to react. Then, because “We all have the impulse to criticize,” we analyze and criticize again.

“Critical Mass” is the culminating performance of Margolin’s “Downtown Theater of the ’80s and ’90s” class. Unlike most performance classes, however, Margolin didn’t audition her students; whoever signed up was in. Lucky for her, this group fits the show effortlessly.

It starts with an honest, endearing introduction from Istel, then switches to Ad Walker’s ’10 introspective thoughts as a theater critic about to watch a show. From there, the scenes flow. Themes of beauty, jealousy, depression, age and pain resonate in Margolin’s fluid but frank script. The lines oscillate between harsh and poetic, funny and tender; then, there is silence — or a poignant musical interlude, sung by Devon Martinez ’11.

Monologues — raw and gripping — keep the play grounded. From O’Hagan Blades’s ’10 portrayal of a 91-year-old grandmother who still feels sexy sometimes, to Dan Amerman’s ’10 turn as a professional basketball player disgusted with fans who boo, and Liba Vaynberg’s ’10 role as a middle-aged mother on the verge of mental collapse, each actor is given a his or her moment to shine. What brings these seemingly disparate characters together is the core subject of their frustrations: the critical edge that cuts and that they use to bite at others.

“I think it’s a replacement for a religion, sometimes, criticism,” says Istel — in “Critical Mass,” we feel sharply the underlying, universal urge to voice opinions. It’s a painful power.

Part of the genius of “Critical Mass” is that it makes us continually reevaluate what we are watching. A play within a play within a play, sometimes Istel the self-aware narrator interacts with Walker the critic; sometimes Walker the critic is Walker the vulnerable man; sometimes Marlene Tempchin ’12 and Keith Rubin ’12 are Mary and Joseph about to have a baby, and sometimes they are starving actors begging for a good review.

Self-aware and proud of it, “Critical Mass” keeps us interested because it never lets us forget that we, too, have all the insecurities that each of the characters displays. We are on the stage with them, and we’re just as afraid of getting a bad review.