Racism is when you make fallacious generalizations about a person due to their race.
Again, I’m not sure if you think that the actor playing martin luther king thing is in fact some kind of great rhetorical victory? You realize that it’s not really compelling in the least, right?
I mean, we already covered it. In that case, one of the actual desirable qualities of the actor, is specifically contained within his actual physical attributes as a result of race, since the part involves playing the role of an actual person from history, who had actual physical qualities you are trying to mimic. Certainly, you could still actually have a white man play such a role, although to do so he would likely need a great deal of makeup to achieve the desired goal.
You aren’t making a generalization about a person due to their race in that case. You are making a statement about the ACTUAL qualities of people of different races, which are objectively true.
To highlight this difference, let’s take a similar case of gender. I can say, “Women are less intelligent than men” and that would be a sexist statement. But I can say, “Women have two X chromosomes,” and this is not sexist at all. It’s merely a statement about an objective observation.
Your Martin Luther King example does not exemplify racism, because it is merely talking about a requirement due to objectively observable qualities, due to a desire to have a physical similarity to an existing person from history.
Yes, I think that perhaps this would be a useful exercise, to find out what exactly constitutes racism in your mind, if saying that you cannot perform a duty which is not specifically centered on being a member of a certain race, due to your race, is not racist.