Justin Cobb (Lou Pucci) is seventeen and still sucking his thumb. It’s hell on his school and sex life. His guru orthodontist (Keanu Reeves in a hilarious cameo) clouds the issue with “hippie psychobabble.” But when Justin is medicated for his attention-deficit disorder, the numbnut becomes a narcissist who leads the debating team to victory. The change shocks his coach (a terrific Vince Vaughn) and his parents (Tilda Swinton and Vincent D’Onofrio), who turn out to be just as screwed up as Justin. Debuting writer-director Mike Mills — remember the name — brings heart and sting to Walter Kirn’s novel. Pucci is an actor to watch: He rides this spellbinder without softening the truths that plague the thumbsucker in all of us.