“Ghost Boy” director Rodney Ascher first learned about Martin Pistorius through the NPR podcast “Invisibilia.” “I remember hearing the story and thinking, ‘This is the kind of story that I would like to tell.’”

The producers of the documentary, premiering Friday at SXSW, reached out to Ascher. They felt the same way: Pistorius’ story about a 12-year-old South African boy who emerges from a vegetative state with no memories, unable to move or speak, but all too aware of his surroundings, needed to be told.

Ascher was keen to pursue the story. Pistorius was inside, a human, but others doubted he was fully aware of what was happening. He took in conversations, like an eavesdropper. He caught TV shows left on for him. In the doc, Pistorius communicates through text-to-voice augmentative and alternative communication software. He spends time reflecting on his journey and the family, friends and caregivers and how...