Q. The idea for “Boyhood” is so original. How did it evolve?

A. 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.

Q. How did the idea come to you?

A. 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.

Q. Your “Before” trilogy looks at how people change and how they don’t over time. Did that figure in?

A. I think it probably goes the other way. This was committed to before the trilogy happened. It gave us a little bit of encouragement to do the second “Before” movie. And that was a scary decision to make. I didn’t know if I had another good story to tell.