Richard Linklater’s new film “Boyhood,” which opened the BAMcinemafest Wednesday at the Brooklyn Academy of Music’s Harvey Theater, follows the life of Mason Evans Jr., from age six through 18. The film hinges on the understated, empathic performance from its star, Ellar Coltrane, whom Linklater met through an extensive casting process.

“I met a lot of smart kids, kids who were going to grow up to be class presidents,” Linklater told Variety. “Ellar wasn’t really like that, he was more ethereal and mysterious. He’s still that way.”

Linklater began filming “Boyhood,” which also stars Patricia Arquette and Ethan Hawke as Mason’s divorced parents and Linklater’s daughter Lorelei as his sister, in the summer of 2002, shooting a few scenes a year for the next 12 years. He had the general trajectory and tone for the film in place since the beginning, but adapted the story to reflect current events, including a scene where Mason and his family campaign for Barack Obama in 2008. “Sometimes we were working on the dialogue the night before,” he said. “Part of the fun was the collaboration with the unknown. It was designed around an unknown future.”

Since he began filming “Boyhood,” Linklater finished eight other films, from crowd pleasers like “School of Rock” to last year’s art house hit “Before Midnight,” so finishing “Boyhood” required not just patience and his unique sensitivity, but also high-level time management skills. “Sometimes it was a bit of a scheduling challenge, but the film never went away,” he said. “I would edit, and attach it to what came before. It was good to have that gestation time. Most films don’t offer you a chance to film a little, edit, and then think for a year. You normally don’t get that, so I tried to take advantage of it.”

After the screening, the cast, as well as Jenny Slate, Amber Tamblyn, Jonathan Ames and Maggie Gyllenhaal, attended the summer BBQ-themed afterparty at Skylight One Hanson, which featured hot dogs, nachos and gourmet salt taffy and caramel popcorn. Before the party, Coltrane, whose parents are “freaky artists, and huge fans of Rick’s,” talked to Variety about how he felt about watching himself grow up on screen.

“I recently have been able to… it’s a lot of me. It’s a very mysterious part of life, the way you change over time. It’s very intense to see that, but also comforting. Recently I’ve been able to remove myself from the film and appreciate it as a whole,” he said. “The first couple of times it was waterworks the whole time.”

(Pictured: Ellar Coltrane and Richard Linklater at the “Boyhood” opening night premiere at BAMcinemafest)