Johnson, a senior at Cass Technological High School, was eating lunch at Small Plates, a popular downtown eatery, when O'Donnell spotted him and asked whether he would be interested in trying out for a film. He took a screen test the next day and won the title role.
When Johnson first saw O'Donnell in the restaurant, he didn't believe it was her, he said.
"I thought it was just somebody who looked like her," Johnson, who had never acted before, told the Detroit Free Press. "I didn't think it was actually her. I mean, what's Rosie O'Donnell doing in Detroit?"
O'Donnell was in town to work on a movie version of a young-adult novel by E.R. Frank. It's about a teenager named America who has spent most of his life lost in the foster care system. He confronts his troubled past with help from a therapist, played by O'Donnell.
O'Donnell told "Entertainment Tonight" that she sensed "a soulful kind of stillness" in Johnson.
During a break in shooting at a former homeless shelter in the impoverished Detroit enclave of Highland Park, Johnson said he remains shocked by his luck.
"It still hasn't hit me," the teen said. "I won't even realize it until it comes on TV; then it will finally hit me, I think."