The emotional terrain of “Blackbird” is established before its two actors even open their mouths.

Director Michaela Johnson’s brisk, precise staging takes place in a workplace’s breakroom, the floor literally piled high with debris in Dark & Stormy Productions’ studio space (someone really likes Cheez-Its).

Una (Sara Marsh) and Ray (Luverne Seifert) stride in and it is immediately clear that she is very tightly wound and he is freaked about whatever it is she wants to say to him. Both of these states are heightened by the debris, particularly plastic bottles that, when the characters step on them, will startle us as surely as if they were land mines.

“Blackbird,” by David Harrower, is one of those two-person dramas that is largely about figuring out what the characters want from each other. Quickly, it becomes clear they’re meeting for the first time in 15 years and she is here among the waxed-paper wrappers and snack-cake boxes to ask a question: “How many other 12-year-olds have you had sex with?”

I don’t think the word “pedophile” is ever used in “Blackbird” but it hangs over the play, particularly when Ray repeatedly insists, “I was never one of them.” But both characters tell lies in “Blackbird” and a big part of the audience’s job in watching the open-ended play is to figure out, based on the finely-gauged performances, when they’re telling the truth. (An unintended part of our job is to unpack the British playwright’s classism, since he seems to think it’s an insult to be a janitor.)

Actors’ choices always have an enormous impact on the plays they perform, but I could imagine them influencing “Blackbird” even more than usual. In fact, I’d be fascinated to see a festival of “Blackbirds,” with a different duo performing the play each night. How might different women handle Una’s attempt to regain some of the power Ray stole from her? How might different men reveal Ray’s pathology?

Seifert’s enormous likability as a performer makes him a brilliant choice for Ray. Initially, he makes you want to believe there’s been some sort of misunderstanding — the way you might if, for instance, you discovered your neighbor abused adolescents — but it also throws the later scenes into relief, making Ray’s protestations hollower and more frightening. Una seems more straightforward but Marsh strips away her teasing intelligence in a painful, emotionally specific monologue that hints she, too, is lying about the direction her life has taken since Ray exploited her.

Both actors in this handle Harrower’s Mamet-y, staccato-rhythmed dialogue deftly, almost as if they’re duetting on a thorny piece of music — a piece that takes on a disturbing life of its own when a late-in-the-play development raises the stakes, revealing what this discussion has really been about all along.