Igby, the conflicted protagonist in writer-director Burr Steers’ dramedy “Igby Goes Down,” is the most confusing, sympathetic and ultimately knowable depiction of destructive adolescent tumult to grace the silver screen in quite some time. Even as he detaches himself from the complications of human contact, he desperately yearns for companionship to replace and eventually heal the wounds of youthful isolation. While he tosses a quip at every concern, he deeply feels the weight of expectation and failure that oppresses his existence.

Much credit for this triumphant exploration of steeled vulnerability belongs to Steers. The newcomer gives his creation words that drip with witty insight and burning cynicism without duplicating the fast-talk neuroses characteristic of a “Seinfeld” episode or a Woody Allen film. But even more impressive than the incisive dialogue is the revelation that these brilliant character nuances and magnificent emotional shifts are brought to life by none other than a Culkin.

Not the burned-out, drug-addled and divorced older one that went bust as soon as his cherubic cheeks flattened and extended, but the younger one, Kieran. The Culkin dauphin has patiently built a resume of understated, natural performances in small, independent gems like “The Cider House Rules” and “The Dangerous Lives of Altar Boys.” “Igby” marks both Kieran’s transformation from child actor to adult leading man and the end of humiliating Culkin jabs that have become an American institution.

While Kieran’s skill almost makes you forget that other actors exist in the picture, “Igby Goes Down” boasts a talented ensemble that merits mention. Susan Sarandon plays Igby’s frigid, vindictive matriarch Mimi — a pill-popping, modern-day Medea who cares more about her own image than her child’s emotional state. Jeff Goldblum portrays D.H., Igby’s ruthless, avaricious godfather who hides his menacing social Darwinist views behind jovial pleasantries and harsh familial logic. Ryan Phillipe is the successful older brother Oliver, a capitalist-in-training at Columbia University who harshly judges Igby’s failure to adapt to his surroundings.

It is no wonder that the film revolves around Igby’s desire to escape his entangling family roots. As our hero so wisely assesses, he’s “drowning in a sea of assholes.” He travels to the Big Apple, where he stays with D.H.’s sensual mistress Rachel (Amanda Peet). During his big city adventures, he bonds with Sookie (Claire Danes), a beautiful fellow wanderer who helps him reconcile his place in an unloving family and a demanding society.

The film’s narrative meanders as Igby himself wanders aimlessly through the city. Further plot complications result when Steers strays from his brilliantly drawn prodigal son. Igby is so believable, so humanly identifiable, that the film falters without his presence. Steers tries too hard to replicate the way last year’s “The Royal Tenenbaums” explored the complexity of each strained connection and twisted bond. Certain peripheral interactions feel extraneous and unnecessary. When D.H. cruelly rejects an insecure Rachel and Oliver sleezily seduces a clueless Sookie, the story and characters lose the momentum and humanity that Igby provides.

With that said, the supporting cast at least manages to successfully complement and explicate Igby’s angst. Goldblum and Sarandon deliver incisive portrayals of monsters ready to lunge from their placid, well-coiffed exteriors. Danes plays her charming misfit with an airy humor and charm. Her calm reflection on an uncertain future nicely balances Igby’s own desperate urgency.

The great thing about “Igby Goes Down” — despite its brief lapses in point of view and narrative coherence — is the way it allows its main character to grow without betraying the honesty of his initial conception. His awkward attempt to hug his detached brother, his reluctant forgiveness of Sookie’s hurtful decision, and his shocking final encounter with his mother all reveal new depth while still reminding us of his conflicted past. In the end, Igby is a little more confident, a little more comfortable, and a little more capable of reconciling his past pain with his future hope.