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In his song “Beautiful Boy,” John Lennon said “Life is what happens to you while you’re busy making other plans.” For a beautiful boy named Ellar Coltrane, life is what happened while he was making a movie.

Starting when he was 7, every summer for a dozen years, Coltrane spent weekends on a unique production with director Richard Linklater. At the end of 12 years, the secret project bloomed into “Boyhood,” the closest thing to a lived life that fictional cinema has yet produced.

Like life, this long movie unspools in mundane moments. And as with so many American kids, Mason (Coltrane) takes the fissures in his family for granted. It seems as if half the film takes place in the back of a car as his divorced mother (Patricia Arquette) leaves another abusive relationship or his out-of-town father (Ethan Hawke) arrives for another weekend adventure with contemplative Mason and hyperactive sister Samantha (Lorelei Linklater, daughter of the director).

While the father drives a vintage muscle car, teaches Mason about the latest music and encourages the kids to emote, the mother bears the unglamorous burden of raising them while juggling jobs and night school. Yet one of the advantages of Linklater’s time-lapse format is how a year’s difference can redirect the winds of change. The penniless mother becomes an honored teacher; the maverick father finds himself married into a family of Bible-thumpers.

Unlike the Michael Apted documentary series that started with “7 Up” in 1964, the narrative “Boyhood” proceeds in a straight line, without flashbacks to underscore the passage of time or the ravages of aging. Some of the important moments and recurring themes do not become clear until years later, as when the lonesome mother makes the same mistakes thrice.

Nor are there glowing mileposts to mark significant events in history. Although one of the mother’s suitors is an Iraq War veteran, the movie unfolds from a kid’s-eye view, and the new “Harry Potter” is more important to him than the latest headlines.

The film would be incalculably different if the lead role had been divided between two or three young actors for a conventional shoot. But Linklater’s patience allows us to see a thoughtful personality being formed both on and off the screen. As Mason arrives at college, discovering a cast to co-star in his young-adult adventure, the sun sets on “Boyhood” and a world of possibilities opens up for Ellar Coltrane.

What “Boyhood” • Four stars out of four • Rating R • Run time 2:45 • Content Strong language including sexual references, and teen drug and alcohol use