Everyone has a drug of choice in ''Thumbsucker." For Justin Cobb (Lou Pucci), the beleaguered Oregon teenager at the center of Mike Mills's slight and disarmingly deft human comedy, that drug is his thumb. At 16, with adolescence confounding him on a daily basis, Justin still finds solace in sneaking off to the bathroom or locking his bedroom door to give the old opposable digit a comfort bath. His father rages, his mother coos, the beautiful girl looks askance, and Justin's new age orthodontist urges him to unleash his inner power animal.
Better they should tend to their own obsessions. Justin's dad, Mike (Vincent D'Onofrio), is hooked on dreams of his youthful days as a football star, before an injury sent him directly to retail. Mom Audrey (Tilda Swinton, showing her soft side) is a registered nurse with a crush on a preening TV actor (Benjamin Bratt). And the orthodontist -- well, let's just say that Keanu Reeves makes the merry most of his small role, summoning the ghost of Bill and Ted and creating a surfer-dude health-care professional who's almost as cool as he thinks he is.
''Thumbsucker" is milder than the roisteringly comic 1999 novel by Walter Kirn from which it's adapted, and on the surface it resembles any number of Sundance-ready odes to dysfunctional suburban teenagehood. It's much slyer than it looks, though, not least because Pucci maintains a beautifully droll baseline sanity through all the changes the world throws at him.
First up is Ritalin, prescribed when his teacher, Mr. Geary (Vince Vaughn, playing the sort of easy-going educator who's sure he's seen it all) decides that Justin has attention-deficit disorder. The drug has profound effects, turning the hero from a timid nonentity into an omnivore who can speed read ''Moby Dick" in one night and tear opponents on the debate squad a new one. Is this who Justin wants to be? Even Mr. Geary wonders if he hasn't created a monster.
For a while, the kid revels in his newfound assurance while everyone else tries on their own ill-fitting roles. Hellbent on using others before they can use her, the beautiful girl (Kelli Garner, from ''The Aviator") gives herself a suburban-stoner makeover; ''Thumbsucker" is nicely attuned to the darker shades of protective coloring kids can take. The only character who seems comfortable in his own skin is Justin's hardheaded younger brother Joel (Chase Offerle), and the movie's implicit message is that happiness may simply be a random gene.
The explicit message, pushed a little too heavily in the final reels, is that self-confidence can't be medicated by legal, illegal, or extra-curricular means but, rather, grows with experience from within. ''Thumbsucker" puts most of its bets on the lank-haired Pucci, though, and the actor comes through with a moving and quietly funny portrayal of teenage discombobulation. Mills trusts his characters -- their hopes, disappointments, and necessary delusions -- and because he condescends to no one, he's rewarded with rich and unexpected performances from both the lesser-known young actors and the better-known grownups (except for Reeves -- he's merely a straight-faced hoot).
For a first film made by a noted director of music videos, ''Thumbsucker" tells its tale with unusual visual modesty. There's no showoff camerawork here, even where it might have perked things up. Less surprisingly, the movie is buoyed by a soundtrack by Tim DeLaughter, leader of the Polyphonic Spree, with three spare songs by the late indie-pop artist Elliott Smith. Like so many recent movies, ''Thumbsucker" uses soundtrack pop as aural shorthand for its young hero's inner state, but it doesn't lean too hard on the music. It's just one more thing to help a boy get by until he gets where he's going.
Ty Burr can be reached at firstname.lastname@example.org.