Gus Van Sant is his own boss again. The director who went from making scrappy indie films like “Drugstore Cowboy” (1989) and “My Own Private Idaho” (1991) to feel-good mainstream fare like “Good Will Hunting” (1997) and “Finding Forrester” (2000) has, in recent years, forfeited a healthy portion of his audience in pursuit of something less conventional. The results have been mixed, but they’ve been more interesting than Mr. Van Sant’s attempts to locate Will or Forrester using traditional Hollywood means, or to replicate Alfred Hitchcock’s “Psycho” shot by shot, as he did in his poorly received 1998 remake.

“Gerry” (2002), “Elephant” (2003), and “Last Days” (2005) formed a loose trilogy of cinematic experiments. Each was an entropic countdown to death observed with unnerving detachment, especially “Elephant,” Mr. Van Sant’s coolly aestheticized re-enactment of a high school massacre.

Depending on your point of view, those films were groundbreaking or pretentious, provocatively ambiguous, or annoyingly noncommittal. Even if Mr. Van Sant didn’t manage to coax a point from his careful compositions, he kept a grim focus on his subjects.

That’s not the case in “Paranoid Park,” a skateboarding tone-poem with an awkward, underdeveloped element of film noir. Mr. Van Sant’s latest work gets its teenage milieu right, but, like its withdrawn protagonist, withholds too much to resonate very deeply.

Here the director is clearly more interested in sketching a portrait of modern adolescence than in fleshing out the dark drama at the film’s center. A sizable portion of “Paranoid Park,” which screens October 8 and 9 at the New York Film Festival, is filmed in slow motion, with skater home videos shot in grainy Super 8 mm serving as a kind of visual refrain. Boys on boards glide and soar through the film’s eponymous skate park, a graffiti-scrawled playground of ramps and half pipes in Mr. Van Sant’s home city, Portland, Ore. Getting airborne, the slow-motion sequences suggest, is how disaffected teens transcend the hormone-addled grind of quotidian life, and one senses that Mr. Van Sant could watch their graceful takeoffs and landings all day. His instincts are fine: It’s actually some of the most compelling stuff int he film.

To its credit, “Paranoid Park” presents a convincing protagonist in Alex (Gabe Nevins), a mop-haired cherub with typical symptoms of adolescent ennui: separated parents, science-class drowsiness, a chirpy girlfriend (Taylor Momsen) whom he could not care less about. Mr. Nevins, a non-actor, is not called upon to do much, but everything from his slouchy gait to his mask of indifference — worn, as is often the case in Mr. Van Sant’s films, over a face that could belong to a Botticelli youth — rings true.

Mr. Van Sant used teenage non-actors to similarly authentic effect in “Elephant,” but Alex’s involvement in the gruesome accidental death of a man near the skate park drags the character into complex dramatic territory that the young actor isn’t able to navigate. As a tidal wave of guilt washes over him, he does little more than put his hands over his face. Mr. Van Sant tries to make up the difference with a long, slow-motion shower sequence after the traumatic incident. But one doesn’t know whether to pity Alex for his plight or admire the way the water drips from his locks.

The narrative circles a bizarre and horrifying accident that occurs one night when a security guard spots Alex hopping a freight train. The episode, which involves a body being severed in two, sits uneasily with the lyrical skater sequences and documentary precision of the rest of the film — indeed, it seems to have been imported from another movie. As Alex quietly, almost imperceptibly agonizes over whether to share what he’s seen, the film bypasses the cues of a typical psychological thriller with almost showy indifference. So why even plant those seeds in the first place?

As an anthropologist of troubled young white males, however, Mr. Van Sant has never been better. The flat dialogue, the casual body language, the skater wear — everything about the kids is rendered with impressive accuracy, and without the salacious leer of Larry Clark films such as “Kids” and “Bully.” In one pitch-perfect scene, a group of skater kids is questioned about the accident in which Alex was involved. The film captures this teen caste’s harsh apathy when a detective (Daniel Liu) passes around a nauseating crime-scene photo — and is greeted by a quiet chorus of guffaws.

By the same token, “Paranoid Park” is limited by the pubescent point-of-view to which it so strictly adheres. Alex’s voice-over narration — actually a confession he writes in his diary — helps the film establish an authentic voice, but it comes at the expense of depth. (“I was so screwed. What was I going to do?”) The elliptical narrative may be an attempt to convey teenage confusion and disinterest, but it’s also alienating. You start to wish a governing adult conscience would step in.

Like the rest of Mr. Van Sant’s unconventional recent work,”Paranoid Park” has a morbid streak. But a more ambitious — and believable — portrait of a skater would have abandoned the B-movie element altogether.