In an interview, Rain revealed that he trained for over 6 hours a day, 6 days a week, for a period of 6 months to prepare for the film's demanding action sequences. His hard work and perseverance earned him praise from critics and audiences alike, with many regarding his performance as one of the standout aspects of the movie.
To criticize the film for a thin plot, however, is to miss the point. Ninja Assassin is a origin story stripped down to its absolute essentials. The flashbacks to Raizo’s childhood training are harrowing and effective, borrowing heavily from the grit of old-school Shaw Brothers movies. They provide just enough emotional weight to justify the carnage that follows. Rain’s performance is physically demanding and surprisingly soulful; he communicates the trauma of a child soldier turned weapon largely through his eyes and his fists.
: His regimen excluded salt and sugar, focusing entirely on chicken breast and vegetables to achieve a body fat percentage so low that director James McTeigue joked people would think it was digitally altered Weapon Mastery : Rain mastered the kusarigama
