The latest from Tracy Letts (whose August: Osage County won both a Tony and a Pulitzer) is an odd-couple comedy of familiar form: Arthur Przybyszewski, a burnt-out, emotionally stunted former radical and draft-dodger who now runs the Chicago donut shop his father founded, hires Franco Wicks, an exuberant, uninhibited black 21-year-old with dreams of literary stardom, to work the counter. They banter, hilariously, as Franco tries to draw his reticent boss out of his shell. The first act is comfortable and entertaining. Bill Geisslinger, who last appeared at Artists Rep as a burnt-out middle-aged cynic in the company’s 2009 production of The Seafarer, incorporates some of that character into Arthur, by way of The Dude and maybe Harvey Pekar. Vin Shambry is immediately likeable as Franco, constantly in motion and endlessly curious, part grifter, part eager student. We love him as soon as we see him. Letts knows this and, because he’s an emotional terrorist, abruptly saddles Wicks with implausible gambling debts, collected by a pair of anachronistic Irish thugs borrowed straight from 1970s Mamet, and sends the plot spinning off into unearned tragedy. It’s a silly, self-indulgent move, and most of the second act is disappointing.