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“What you got to do is keep clear who your friends are, and who treated you like what,” says Donny, the dodgy but essentially decent proprietor of a dusty secondhand shop, to Bobby, his slow-witted, eager-to-please proté#gé#.

It’s not far into American Buffalo, David Mamet’s trash-talking Chicago tragedy, that this admonition crops up. By shortly after intermission, of course, it’s Donny who’s wondering which of his friends is least reliable, and by the final scene, poor drugged-out Bobby has taken some fairly rough treatment from people he’d been pretty sure he could count on. Mamet’s brutally efficient character study centers on three losers trying to codify what honor means among a not particularly thoughtful bunch of thieves; the Source Theatre’s production, an unpretentious but powerful affair directed by Joe Banno (the company’s artistic director and the Washington City Paper’s opera critic), thoroughly captures the depth of their failure and the sad claustrophobia of their shabby side-street world.

“There’s business and there’s friendship,” Donny (Tim Carlin) tells Bobby (David Lamont Wilson) in that same early exchange, but in a world where business is almost always shady and friendship doesn’t necessarily imply trust, “things are not always what they seem.” Bobby’s reply to this piercing bit of blue-collar wisdom is an offhand “I know,” but of course he doesn’t, and neither does Donny when it comes down to it: The play’s action, and its tragedy, turns on the trouble he has sorting out truths and half-truths from the petty greed and paranoia that poison the shop’s air when Teach (Rick Foucheux), his partner in a planned burglary, suspects Bobby of a double-cross.

That all this grief is inspired by a nickel—a rare buffalo nickel, more valuable than Donny realized when he sold it to a local collector—seems to hint at a kind of authorial disgust for the priorities and postures Donny and Teach voice as they justify recovering the coin (and stealing the presumably valuable collection they believe it’s gone to join). And to a point, that view may be valid: They’re both whiners at root, sneeringly class-conscious, prone to peacocking about their own tatty dignity, and jealously contemptuous of anyone more successful. But listen more closely and you’ll hear the author connecting the dots between these two nobodies and the people in the good seats; self-absorption and suspicion aren’t in short supply in the glorious Republic, after all, and Mamet’s is an equal-opportunity revulsion.

For all their talk about clear distinctions and facing facts, neither Donny nor Teach is especially savvy about human nature, even if Donny does seem to have decent instincts. Carlin, most recently the sweet, sad airport suitor (and other characters) in Banno’s Love and Yearning in the Not-for-Profits at Theater J, brings some of that portrayal’s soft-edged sadness to this part; he’s basically good-natured and essentially humane in his impulses, if ultimately unable to govern his fears.

Not so Teach: He’s written as a half-assed operator with more bluster than brains, and with this performance Foucheux, so thoroughly dissipated and dark-hearted in Woolly Mammoth’s Heaven last season, adds another vividly drawn portrait of failure to his portfolio. He’s a seedy, surly, frayed-at-the-seams bit player in a game that even he recognizes as low-stakes, the kind of sour-smelling, sleep-deprived wreck who’d carry a gun just for the ego boost; Foucheux gets so completely into the part that the Source auditorium, crammed to the rafters with crap apparently cadged by designers Greg Mitchell and Elsie Jones from every prop shop and thrift store in town, seems to reek of stale sweat. Banno takes a few directorial swipes at Teach’s dignity, too, subjecting him several times to minor humiliations (a recalcitrant recliner, a laughable paper hat in a driving rain) of precisely the sort that inspires the character’s outsize Act 1 rant.

If that operatic expression of ignorant outrage is the play’s only real set piece, the language throughout is Mamet’s unmistakable brand of malign music: circular and funny and foul—even in this early effort, profanity serves as a kind of reflexive punctuation, though it’s true that the conversational rhythms in this ’70s-vintage play are nowhere near as halting and repetitive as the complex riffs of later shows. Bobby’s opaque ramblings excepted, sentences here tend to be complete, even if the thoughts behind them aren’t. Malapropisms pepper the script, signals of how thoroughly Teach’s reach exceeds his grasp: “God forbid something inevitable occurs,” he blusters in one of the circular arguments that prove how unfit he is for even the petty crime he’s trying to plan.

Wilson’s Bobby—both his loose, agreeable air and the fact of his dark skin—creates a layer of conflict that may not have been present in the original production, in the Stage 2 series at Chicago’s Goodman Theatre, which put a young William H. Macy in the part (though presumably somebody has since cast a black actor in the role). Certainly one of Teach’s later and more violent explosions, underscored with a racial epithet that wouldn’t be quite as pointed with three white actors onstage, speaks in this production to how thoroughly threatening he finds Bobby—his best friend’s new best friend. And the exchange emphasizes, more than it usually might, that Teach’s suspicions about Bobby are based as much on ghetto stereotypes as on the evidence he thinks he’s seen.

What makes American Buffalo a terrific play is that many in the audience, especially those who haven’t seen it onstage or at a movie house, will walk to the brink alongside Donny, wondering if Teach might not just be right about Bobby. What makes it a great play, a piercing and powerful study of human frailty, is that far too many of us will keep walking over the edge with him. CP