Charlize Theron had eight personal trainers to help her master her intensely physical performance. She also trained with Keanu Reeves, who was preparing for John Wick: Chapter 2 (2017).
Before filming resumed, James McAvoy broke his hand on the set of Split (2016) and had to endure all his action scenes with his injured hand.
The pivotal fight on the marble stairs presented one of the most challenging sequences to film. Charlize Theron was actually slammed against a (padded) wall but her tumble down the stairs (also padded to look like marble) was executed by Canadian stunt woman Monique Ganderton.