It starts, as it always does, with what pretends to be a valid argument.
"Hey," one observer mentions on Reddit or 4chan or whatever depressing part of the web where decency goes to die, "what's with that new Star Wars trailer? The first thing we see is some black guy in a Storm Trooper outfit. But how can that be? Storm troopers are all clones."
Then someone else points out that no, that was a thing that happened during the Clone Wars (as established in Episode II: Attack Of The Clones, and yes, I'm embarrassed to know this), but the Clone Wars were already over by the time the original trilogy started, and the Storm Troopers were just guys recruited by the Empire, and somebody else says, No, man, you're wrong, and the conversation continues, and pretty soon it becomes obvious that the original poster wasn't so much complaining that the black guy playing a Storm Trooper conflicted with what he saw as the series' continuity as he was...complaining about a black guy being in a Star Wars movie.
The conversation deteriorates, as again, it always does, with claims that a black actor having a prominent part is a capitulation to the forces of "political correctness" which descends to watermelon and fried chicken jokes and undisguised racism of a force and virulence that you thought surely would surely no longer exist in this day and age.
But of course, it does exist, as one is reminded on a daily basis when reading any comments board on the events in Ferguson, or the ongoing sexual assault claims against Bill Cosby, or pretty much anything to do with Barack Obama, whose very legitimacy as a president and an American citizen is challenged by bullies online every single day. But not because he's black, God knows. You can tell, because these claims are frequently preceded by the claim, "I'm not racist, but..."
And then it starts, as it always does, with what pretends to be a valid argument.