There are few filmgoing experiences more frustrating than watching a decent, thoroughly enjoyable picture dissolve into a puddle of unpalatable goo in the final reels. Tom Hanks' directorial debut, for most of its running time, is as genial and unpretentiously pleasant as are most of the characters he plays, but when Hanks attempts to add some dramatic heft in the waning moments, you're liable to develop a headache from repeatedly rolling your eyes, if not disturb other patrons with unsufficiently hushed interjections of "Oh, christ and "Dear god." The film also suffers somewhat from its similarity to a superior one; That Thing You Do! tells more or less the same story as did Alan Parker's adaptation of Roddy Doyle's novel The Commitments -- teenagers form band, achieve a limited notoriety, squabble, and disintegrate at the peak of their success -- but Hanks' tale is considerably less textured and not half as hilarious. What makes it such a low-key, understated hoot, until it collapses, are a handful of winning performances; the keen attention to period detail; and Hanks' goofy sense of humor, which is clearly evident in his screenplay -- as when the band's guitarist, played by Steve Zahn (who plays virtually the same character in Richard Linklater's upcoming subUrbia, only drunker and sans guitar), corrects a fan who mispronounces the band's ludicrous name, The Oneders (Wonders) as The Oh-Need-ers: "That's 'The Oh-Ned-ers.'" (Okay, that joke doesn't really work in print, but it made me laugh for a long time.) In addition to Zahn, eerie Hanks act-alike Tom Everett Scott, brooding Johnathan Schaech (The Doom Generation), and Hanks himself, That Thing You Do! also stars Hot New Thing Liv Tyler, who has appeared on the cover of every magazine short of Guns & Ammo but still has not demonstrated to my satisfaction that she can actually act. To be fair, however, the vapid character she's saddled with here would make even Kate Winslet look dim; furthermore, Tyler has the dubious honor of delivering the single worst line of dialogue I've heard in a movie theater all year: "Shame on me for ever kissing you with my eyes closed so tight." (Dear god.) We're given no reason to care about this obligatory girlfriend, who spends most of the movie standing dewy-eyed in the wings mouthing the words to the title song along with her fella, so it's an unpleasant surprise when she suddenly wanders into the narrative spotlight in the last twenty minutes, at the apex of a banal romantic triangle that stops the picture cold. It never recovers. Free advice for Hanks, if he wants it: next time (1) be less ambitious, and (2) give Peter Scolari a bigger part.