If not exactly a secret (Linklater and star Ethan Hawke have alluded to it repeatedly in interviews over the years) “Boyhood” has nonetheless sounded like a kind of creative mirage: an ambitious pet project hinging on a stunt seemingly too clever to be true. Shot over a period of 12 years, using the same four principal actors throughout, it”s as literal a coming-of-age tale as has ever been conceived for cinema, with the fractured Texan family at its center growing up right before our eyes. Protagonist Mason (Ellar Coltrane, in the most long-gestating breakthrough performance of all time) enters the film a bright, thoughtful six-year-old; nearly three hours later, he leaves it a still-precocious young man on his first day of college.

His is the most radical, unnerving transmogrification, though it”s a film of several: even Ethan Hawke, playing Mason”s playful, semi-absent man-child of a dad, seems ageless for several years until, suddenly and quite strikingly, he isn”t. The title, indeed, is a bit of a misnomer, unequal to the project”s generous, wildly expansive scope. Mason's boyhood is but one arc in a film that observes how adults and children alike are forced to build and rebuild lives, personalities, families and homes, subtly redefining themselves as often as they more visibly change address, relationship status or hairdo. Now that “Boyhood” is with us, it seems positively bizarre that Linklater – whose work rate has been more or less consistent over the years, even if the work itself has not – could have kept it stewing so steadily on the back burner all this time as he busied himself with the lesser concerns of “The Bad News Bears” or “Me and Orson Welles.” How does work on this scale not consume its creator?

Yet it”s the casual ease of “Boyhood”s” construction, its lack of a specifically lofty artistic objective, that makes it so effective: life at any stage isn”t lived according to a script, and Linklater”s loose, permeable narrative does its best to reflect that. Its closest cinematic forerunner, Michael Apted's ongoing series of “7 Up” documentaries, has of course been defined entirely by its subjects” life choices, though the application of the concept to fiction – and compressed into a single feature film, to boot – invites a very different question of creative process. As with Francois Truffaut”s landmark Antoine Doinel series, how much “Boyhood” has been led by Linklater”s imagination and how much by the physical and psychological development of its own stars is all but impossible to determine.

Wherever the meeting point was, however, it was the right one. This yearly check-in process could have yielded chilly, laboratory-style results, but  the film emerges as lively, messy, spontaneous – palpably and propulsively a movie rather than a showy experiment. It”d be mesmerizing even as a more static exercise, but “Boyhood” is a testament to just how much stuff happens even in purportedly unremarkable lives. The shape and nature of Mason”s home life, for starters, is in constant flux – though the two constants are his mother Olivia (Patricia Arquette), a spiky divorcee whose no-nonsense smarts don”t extend to her catastrophic taste in men, and his older sister Samantha (played by Linklater”s own daughter Lorelei, with an early aptitude for testy irony).

Olivia”s decision to pack up the kids for Houston and re-start college is a decidedly mixed blessing. Studying psychology brings her the life she feels she deserves; marrying her boorish, abusive professor and merging her family with his certainly does not. The ensuing scenes between them play as landmines of conflict and, ultimately, plain terror in the middle of this otherwise rolling, sprightly chronicle. There”s further relocation and one more unwelcome stepfather to come, but Mason – his sensitive, laidback demeanor serving as oil to the water of any would-be authoritative paternal figure – learns to roll with the punches, acquiring an inner resilience that belies his soft-shell exterior. Watching Mason grow into his personality (part of it acquired, most of it rooted firmly in his younger self) is fascinating; with his liberal cool and quiet curiosity, he”s very much a 21st-century child, though he doesn”t seem molded by contemporary youth culture so much as lucky to be entering it at the right time.

One wonders how much of this character study has been influenced by Linklater”s own shifting relationship with his young leading man, who can”t have been cast with any degree of certainty over who or what he would become. That”s a thrilling risk for a filmmaker to take, and Coltrane rewards his director”s gumption by blossoming into an actor of genuine charisma, blessed with lanky physical grace and drolly behind-the-beat delivery. As he enters adulthood, it”s not hard to see how Mason might fit into the director”s 1991 debut “Slacker,” even if that film, somewhat alarmingly, precedes his entire life. Whether by accident or design, Linklater has stumbled upon a perfectly Linklater-esque figure for his children's generation.

It”s compelling, too, to watch adult actors craft their characters on such a long-term basis: as in “Before Midnight” last year, the greying and fraying of Hawke”s hipster-dreamer persona seems part self-deprecating and part unconscious. Arquette, meanwhile, is tremendous, charting Olivia”s shift from overwhelmed single mom to sleek suburban bohemian to wry, hardened survivor – sliding back and forth between those poles as circumstances dictate – with unflagging good humor and ferocity.

The impressive continuity of the performances is matched by that of Linklater”s filmmaking: balletically edited with barely a trace of the disjointed process behind it, the film gives every impression of having been conceived and built as a single entity, so complementary and mutually fulfilling are its narratives and rhythms. Only in Mason”s concluding, graduating year did I feel the length of the enterprise: it seems Linklater knew what he had on his hands by this point, and milks its significance ever so slightly.

There is, of course, an incalculable time-capsule quality to the finished product, as a dozen years” worth of popular and political culture is reflected in sundry incidental details on screen – be it a faddish YouTube video, a naff bubble-style iMac or the thankfully short-lived assimilation of the word “kewl” into the everyday lexicon. Linklater also has no need for chronological title cards when his typically astute, witty pop soundtrack does whatever work his actors” sharply evolving faces cannot: here”s a film to remind us that Soulja Boy was once a thing, even if it can”t remind us why.

For any viewers much older than Mason is by the end of proceedings, “Boyhood” is at once a lovely and sobering snapshot of just how much life can be lived in a relatively short space of time. The film begins, as it happens, in a year when I was starting college myself; watching it, I could see where certain generational batons had been passed, in some cases into worthier hands. Improbably vast and beguilingly small, “Boyhood” is a film built on the past, but restless for the future. I wonder if Linklater can possibly bring himself to leave it alone.