Thawack! Wham! Bang!It’s 1941, and Thurgood Marshall (Chadwick Boseman, “Get on Up”), the 32-year-old lawyer for the National Association for the Advancement of Colored People, is whacking, hard, a billy club — dubbed “the n---- killer” by its constabulary owner — on a southern courtroom table where a black man sits.Everyone in the courtroom (and the movie theater) is rattled, and we flinch along with the accused at the force of the blows.Marshall has made his point that there is …