You don't have to look very far to find thoughtful articles on the subject of the Magical Negro in literature. In addition to the one I first linked to (Stephen King's Super-Duper Magical Negroes by Nnedi Okorafor-Mbachu) here are two others mentioned in an overview at wikipedia:
3 Vectors of The Magical Negro
Movies' 'Magic Negro' Saves the Day - but at the Cost of His Soul
I'm going to summarize, rather ruthlessly, what I see as the main points and arguments:
There is a recurring stereotype in literature and film which has been called the Magical Negro or, as Spike Lee rephrased it, the Super Duper Magical Negro. The MN is more a plot device than a real character. In a storyline where a white person is in desperate need of direction and guidance, the MN is the character who appears, often out of nowhere, to provide what is needed.
The main characteristics of the MN are:
he or she is isolated, with no observable family or community of their own and no backstory or conflicts;
the only purpose of this character is to provide the support the white protagonist needs;
the MN often has supernatural or magical powers;
these characters are benign or supportive, with no personal stake in the outcome of the story;
the MN either slips away quietly once his or her work is done, or is sacrificed so that the protagonist can prevail.
The articles listed above all list books and movies which rely on this stereotype. Some examples:
Noah Cullen (Sidney Poitier) in the film The Defiant Ones (1958)
Dick Hallorann (Scatman Crothers) in The Shining (King, 1977)
John Coffey (Michael Clarke Duncan) in the serialized novel The Green Mile (King, 1996)
Cash (Don Cheadle) in the film The Family Man (2000)
Bagger Vance (Will Smith) in the film The Legend of Bagger Vance (2000)
The problem with this kind of characterization is that it perpetuates a lot of negative stereotypes. The kindly, selfless Negro whose primary purpose is to support the white protagonist and make sure that he or she finds a solution to a tough problem. Or you could see it like this: black characters have a far more limited range of roles to play in our stories, and one of them is the Magical Negro -- which looks, on the surface, like a positive portrayal but which really is nothing more than second ranking to the ever present and pervasive white characters.
Storytellers often reach for plot devices. A stereotype is a plot device, and used to excess, the hallmark of a lazy or unimaginative writer. It's far easier to take a stock character off the shelf and dust it off than to sit down and construct a real personality with real wants and needs and motivations. And of course, to some degree stock characters are a necessity. A writer who takes the time to put together a full backstory for every minor character who flits on and off the page is a writer who will never finish a novel.
Thinking about this, I tried to come up with stereotypes that could be compared to the MN, and it occured to me that the MN is an innovation based on the stock character of the interfering angel which showed up in a lot of movies in the forties and fifties and still shows up now and then. Clarence the Apprentice Angel in It's a Wonderful Life; Dudley the angel (played by Cary Grant) in The Bishop's Wife; Here Comes Mr. Jordan, Heaven Can Wait, Wings of Desire, What Dreams May Come.
So now I'm wondering if there aren't multiple issues here. One has to do with a certain type of story that depends on a stock character, one with the way African Americans are portrayed, and the third with the variety of roles available to African American actors.
Would it be possible to retell any of the stories in the first list above and switch the race roles? Putting aside the issue of historical fiction for the moment, think about this: you're casting for a remake of the movie The Family Man. In this movie Jack Campbell (Nicolas Cage) is the stereotypical Wall Street high powered business executive, the guy who works Christmas because he wants the deal and his love life has mostly to do with paid professionals. On Christmas Eve he goes into a convenience store and finds himself in the middle of an escalating situation involving guns and a very rough, street wise black man (Don Cheadle). Jack Campbell takes a risk and intercedes and the situation is defused. Then he runs into Don Cheadle out on the street, and it turns out he's not what he first appeared to be. He offers Jack a look at what his life would have been like if he had married his college girlfriend and taken a different path -- one that involved kids and a house in the suburbs.
Don Cheadle comes across as a very rough customer in this movie and Nicholas Cage as very composed and sophisticated. Could you switch the roles? I think this could be pulled off, as they are both strong actors. In other stories it gets a little more difficult. In Bruce Almighty, you've got Jim Carrey as the white guy in need of guidance and Morgan Freeman as the God who gives him the powers he thinks he wants.
Could you switch those roles? A white God and a black man in need of guidance? Harder to pull off.
It's easier to rationalize a two dimensional character who is not of this world. What is there to know about an angel's backstory? Is there any doubt about God's motivations? The problem is when the character is grounded in the here and now, but is treated like an abstraction. The Never-Mind-About-Me Negro, the Magical Negro, the Negro without needs or wants of her own.
All this brings me to the topic of John Coffey in The Green Mile. Because while on the surface he would seem to be an example of the Magical Negro, there's more going on about his character. He's certainly part MN, but King seemed to be trying to move beyond the stereotype. And I'll put that down in more specific terms tomorrow, after I've done some work and seen to the Mathematician. Who, by the way, does indeed need surgery.