Why couldn't a white man play Martin? Probably would do a better job at it than a black man. Anthony Hopkins would be great.
The answer is completely ridiculous, not the question. Here is why:
1. He does not have the balls to say what's on his mind, namely: "getting sick and tired of blackwashing. Fuck you!!"
2. He claims that a black guy wouldn't make sense to the story. What story? A secret police man can't be black realistically? I bet in reality they do exist, in quantum of solace there was by the way a black guy secret police man on the motorcycle. Secret policemen are just assassins according to the story after all, bond is not working on unlocking the greatest mysteries in modern physics or anything like that.
3. His comparison with MLK is dumb as rocks. The entire theme of such a film would be race. But even still I see no issue with a white man playing MLK, nor why white men and women couldn't be playing the mobs of angry black folk. It would make for great experimental cinema. In Africa they make movies about the life of Jesus with all black casts. It's about the story after all, not about everything being completely accurate.
