Fittingly enough for a film about a long, unhurried process of discovery, it wasn't until near the wrap of production that Richard Linklater decided he would call his 16th feature "Boyhood." That was in the summer of 2013, more than a decade after he and his cast and crew had shot the first frames of their movie about an East Texas kid named Mason Evans Jr. and his journey through childhood and adolescence. Seeking a title that would suggest not only the picture's narrative scope but also its lengthy shooting history, Linklater settled on "12 Years" -- a seemingly perfect choice, at least until the writer-director discovered there was a similarly named, soon-to-be Oscar-winning prestige picture on the horizon.

"I was like, not '10 Years a Slave?' Not '15 Years a Slave?' Are you kidding me?!" Linklater says with a laugh. "I was like, OK, the world is telling us to stay out of numerical titles."

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Another early possibility was "Childhood, Boyhood and Youth," a nod to Tolstoy's autobiographical 1852-56 trilogy of novels. But it was an abbreviated version, "Boyhood," that wound up on the finished film, even if it still strikes Linklater as somewhat misleading -- and not just because his young star, Ellar Coltrane, whom we first encounter at the age of 6, is no longer a boy but a handsome lad of 18 by film's end.

For although "Boyhood" is in some ways the ultimate coming-of-age story, told from a gradually evolving, deeply immersive child's-eye perspective that inevitably transports us back to our own lost youth, it is no less the story of Mason's family -- a microcosm of an archetypal splintered American brood. The pivotal roles of Mason's parents, who are already divorced when the film opens, are portrayed by Ethan Hawke and Patricia Arquette, who, along with Coltrane and Lorelei Linklater (the director's daughter, who plays Mason's older sister), made a long-term but non-binding commitment to the project in 2002.

While the shoot progressed in three- or four-day increments over the next dozen years, the annual process of brainstorming took months, during which time the actors participated in Linklater's writing process, filling in his broad narrative outlines with their own dialogue and personal experiences of childhood (and, for Hawke and Arquette, parenthood). Free-form as it was, every year this creative alchemy resulted in one more 15-minute chunk of the script, albeit sometimes pulled together as late as the night before shooting.

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"I was aiming for reality. I never wanted it to feel written, but it was," Linklater says in an interview at his Austin Film Society offices, in advance of the picture's July 11 domestic release through IFC. "I was always just taking the temperature of these four people where they were at, and where they were going. It was an ongoing collaboration. But the biggest collaborator here, looking back, was time."

Whatever else it may be -- an epic cinematic bildungsroman, an aughties pop-culture time capsule, an apt demonstration of Jacques Rivette's maxim that every film is a documentary of its own making -- "Boyhood" feels above all like that great movie rarity, a fully realized experiment. Fluid, funny, melancholy and wise, it unites the youthful, form-busting spontaneity of Linklater's early work, like "Slacker" and "Dazed and Confused," with the casual profundity and masterly assurance a filmmaker attains only with age.

By the time the picture made its world premiere at Sundance (nearly a quarter-century after "Slacker" screened there in 1991), the waves of rapture flowing through the Eccles Theater seemed to signal more than just an audience swooning for another movie. Even coming from Linklater, who had brought the Park City crowd to its feet just a year earlier with "Before Midnight," this haunting new work was clearly something special -- a film the likes of which the audience had never seen and indeed might never see again.

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At 53, Linklater has built a quietly unassailable case for himself as either the most experimental mainstream artist or the most mainstream experimental artist now at work in American movies. Possessed of an essentially comic temperament, he has become an intimate, emotionally generous observer of everyday life whose finest films -- among them "Boyhood" and the romantic trilogy of "Before Sunrise" (1995), "Before Sunset" (2004) and "Before Midnight" (2013) -- are not just relationship studies, but also compelling exercises in form and content.

Taken together, "Boyhood" and the "Before" pictures amount to one filmmaker's monumental reflection on the passage of time, the irretrievability of the past and the uncertainty of the future, and the two projects are more closely linked than even their shared themes would suggest. While Linklater and Hawke had long toyed with the idea of revisiting the characters from "Before Sunrise," it wasn't until 2002, when they committed to the 12-year sprawl of "Boyhood," that they felt sufficiently emboldened to make "Before Sunset," a film that Linklater still describes as "maybe the scariest thing I've ever done." He quickly adds, "It's good, in art, to do those things you're a little afraid of."

(Photographed by Dan Winters for Variety)

If the "Before" movies are essentially Linklater's riff on Rohmer, each one an endearingly loquacious two-hander played out against an idyllic Old World setting, then "Boyhood" is unmistakably his tribute to Truffaut, who directed perhaps the greatest movie ever made about restless youth, "The 400 Blows." Similarly, the French master's extended collaboration with actor Jean-Pierre Leaud as Antoine Doinel feels like an early template for what Linklater and Coltrane have pulled off here.

To the uninitiated, those might sound like lofty reference points to attach to Linklater, a laid-back Austin native who goes by Rick and punctuates every other sentence with "yeah" or "you know"; and who, during our interview, sometimes himself resembles an overgrown kid with his T-shirt, shorts and unruly mop of hair. But no one familiar with the filmmaker's work would be surprised by his penchant for odd philosophical digressions or his aw-shucks erudition. It's the same stealth intelligence at work in his movies, which often conceal an unusual narrative and formal ambition beneath their shaggy, unpretentious charms -- a subversive streak that has set Linklater apart from some of his more bottom-line-oriented contemporaries.