But Linklater shrugs. To him, the movie patiently made by living its own subject, time isn't anything audacious.
"It was no different than life itself," said Linklater in a recent interview with Coltrane. "When people say, 'Oh, it was really risky,' well, I don't know. Compared to what? You're either alive or you're not. It was optimistic that we would all still be here 12 years from now. Statistically, it was probable."
What was less likely was that Linklater would pick the right 6-year-old boy out of auditions, only guessing at how adolescence and puberty would shape his star. Though it was a pact Coltrane (and his parents) made when he was barely conscious, Coltrane never waived. "I'm along for the ride," he says, upbeat that the film "this little private thing," Linklater calls it is finally coming out Friday.
"Boyhood" is about an Austin, Texas, family of four: a boy named Mason (Coltrane), his sister Samantha (Linklater's daughter, Lorelei) and their divorced parents (Ethan Hawke and Patricia Arquette). The project conceived by Linklater to chart Mason from first grade to high school graduation was so unusual that none of the cast could sign contracts for its duration. (The term limit is seven years.)
The movie is fiction but its power is as a document to life. To represent aging, Hollywood has long employed a barrage of techniques, but those tools seemed artificial to the independent filmmaker whose "Before" trilogy (which checks in on a romance every eight or nine years) also chronicles the unexpected twists of time.
Instead, "Boyhood" was made in just 39 shooting days, albeit ones spread out annually over a dozen years. Linklater edited as he went, rewriting to tweak the largely pre-planned story to include changes in Coltrane and the wider world.
Coltrane effectively grows up on camera. Watching himself through his early teenage years, Coltrane acknowledges, can be brutal: "It's like staring into my own soul."
"Everyone kind of wonders how you change, day to day, much less over years," says Coltrane. "You have this idea of you as a child being very different from who you are when you grow up. But really, that's one of the most terrifying parts of this is that I'm the same person."
And that, in some ways, is the revelation of "Boyhood" that while we all evolve and mature through time, we are essentially who we are, both child and parent. Our lives aren't defined by the big dramatic moments usually highlighted in movies, but flow more naturally.
"It's kind of comforting in a way to see it all together like that," says Coltrane. "It kind of forces me to accept myself, for better or worse."
Much of "Boyhood" is keenly observed moments like Mason shoving rocks in a pencil sharpener or being annoyed by his sister singing Britney Spears (music moves chronologically in the movie). Mason's mom goes through a series of unsuccessful relationships; his dad drifts in and out, taking the kids bowling or to post Obama election signs.
"Everything about life could be incorporated into this movie," says Linklater.
Michael Apted's "Up" documentary series has followed its subjects' lives for decades. In the fiction realm, Francois Truffaut explored his character for over 20 years. But "Boyhood" is unique in condensing so much time in such short increments, and it can be astonishing to see age slowly etch itself on the faces of Coltrane et al.
"Film is a powerful recorder of the reality in front of it," says Linklater, who references early silent films' recording of daily life. "That's what makes it such a magical medium."
Coltrane now finds himself like Mason leaves off contemplating his next steps. Having been homeschooled by his parents, he's not sure yet about college, but says, "I just crave knowledge."
Though Linklater worried about the "psychic overload" he was potentially putting on Coltrane and his daughter, Coltrane has apparently graduated from the project an open-minded, philosophical young man eager for adventure even if his path is unclear: "I don't know if I'm an actor, really," he says.
"To me, probably the most satisfying was the evolution of him, becoming more and more of a collaborator," says Linklater.
There aren't plans for a sequel, but Linklater knows better than to predict the future.
"Who knows?" he says. "Life unfolds, obviously. It's endless possibility."