In an interview Linklater discusses Boyhood starting with how the idea came to him:
In a flash. I was kind of problem solving. I had a problem in that my ideas for a story were dispersed over the years. You can’t make a film that way. If you cast 12 people at different ages in the role, audiences wouldn’t believe it. You can’t keep changing the kids out, and I felt that limitation. I had almost given up on the idea when it hit me: What if we filmed a little bit every year? That became the challenge, to show this family slowly, gradually changing.
It’s a crazy way to make a film, admittedly. But it was kind of the only way to tell this particular story. I was trying to say something about growing up, who we become. Are we the same people at 18 that we were at 7? It became the only way to tell the story.