John Boyega did not see the racist graffiti in the changing room until the cameras were rolling, so his angry reaction was genuine.
For the scene where Leroy Logan visits his beaten-up father in hospital, Steve McQueen had the set cleared out of unnecessary crew so it was essentially just him, John Boyega and Steve Toussaint. Before filming began, McQueen took Boyega aside and told him something very personal from his own background to help Boyega dig deep for his performance.
This film was already in the can before the global Black Lives Matter demonstrations took place in the summer of 2020. John Boyega made headlines that summer with his impassioned speech at one of the London protests.