https://en.wikipedia.org/wiki/Boyhood_(film)#Production

In May 2002, film director and screenwriter Richard Linklater said that he would begin shooting an untitled film in his home city of Houston that summer. He planned to assemble the cast and crew for a few weeks' filming annually for 12 years. He said: "I've long wanted to tell the story of a parent-child relationship that follows a boy from the first through the 12th grade and ends with him going off to college. But the dilemma is that kids change so much that it is impossible to cover that much ground. And I am totally ready to adapt the story to whatever he is going through."IFC, the film's distributor, committed to a film budget of $200,000 per year, or $2.4 million over the 12-year shooting period.
Linklater hired the seven-year-old Ellar Coltrane to play the boy.The cast could not sign contracts for the film due to the De Havilland Law, which makes it illegal to contract someone for more than seven years of work. Linklater told Hawke that he would have to finish the film if Linklater died. Boyhood was written over the shooting period, with all major actors playing a part in the writing process; scripts for certain scenes were sometimes finished the night prior to shooting.