Casey Affleck had been working on a script about a parent and child for years. He’d written scenes about a bedtime story and an argument about taking a trip into town and things seemed to be going well. As the father of two boys, Affleck had defaulted to writing the child as a son. But then, deep into the process, his boys came back with a request: They didn’t want it to be about them.

“They were pretty adamant that they did not want it to be a father-son story,” Affleck said. “I had already put in a lot of work and I was like, don’t do it ... ”

But, despite the heartburn of having to rework the story again, he caved and made the child a girl, and it ultimately helped the story. He’d been looking for stakes in this parenting saga and now a genre construct was possible. What if a disease had wiped out all of the women, except this man’s daughter?

“This was about someone who was very nervous about protecting their kid. I wanted to make the stakes of that as big as possible so it wasn’t just like, I’m protecting them from having a bad fourth grade experience,” Affleck said. “It’s like I’m protecting them from an entire world who wants to kidnap them.”

The result is the slow-burn dramatic thriller “Light of My Life” now playing in theaters and available on demand. Affleck directed, wrote and stars in the film, which premiered earlier this year at the Berlin Film Festival. It knowingly echoes modern classics like “Children of Men.”

He and casting director Avy Kaufman went on an epic search to find the right girl to play his daughter, Rag, and discovered a little-known Canadian actress named Anna Pniowsky who fit the bill perfectly.

“I was still a little girl. I think I was 11 when I sent my tape in,” Pniowsky, now 13, said. “I wasn’t expecting anything to happen. I didn’t get a lot of roles back then.”

Since then, her profile has risen with roles in the Hulu show “Pen15” as one of the popular girls and “The Hot Zone.” And she fell easily into her role as a pre-teen who still acts out and defies her fathers’ rules even in this bleak and dangerous reality. Critics have hailed her performance, much of which is spent in “disguise” as a young boy, as a breakout.

“She didn’t need a whole lot,” Affleck said. “She was just good at doing this naturally. Sometimes I felt like if I interfered too much in the scene or the natural progression of the scene that I was just getting in the way.”

Together, Affleck and Pniowsky have a light-hearted rapport where you can see the seeds of why their father-daughter dynamic works. They can’t even agree on whether or not Affleck gave her anything to prepare beforehand.