Josh Rivera Takes the Lead in American Sports Story

I’m trying to slim down now, ” says Josh Rivera half-jokingly. “Hollywood loves the lean boys. Timmy Chalamet is working his butt off.” We’re at the rock-climbing gym Brooklyn Boulders in Long Island City on a July afternoon. Rivera, who packed on nearly 30 pounds of muscle to play Aaron Hernandez, the late star Patriots tight end who was convicted of murder, bulges out of his loose cutoff beige shirt and gray shorts.

Rivera was 24 years old...