James Earl Jones received his only Oscar nomination for portraying Jack Jefferson in The Great White Hope.
Jack Johnson is certainly a difficult character to portray since in real life he always carried a larger than life manner to himself, and never strove away from being flamboyant about his skill as a boxer or his ability to anger white people. James Earl Jones at the beginning of the film tries to imitate this aspect of Johnson right down to his very distinct smile. James Earl Jones actually does do a good job at this aspect of Johnson's character, and Jones has the right charisma and presence in these scenes. He surely is a little annoying, and very flamboyant but that does indeed reflect the way of the real Jack Johnson, and Jones does a very good job suggesting the whole way and manner of Johnson.
Now one of aspects of the film that is heavily focused on is Jack's relationship with a white woman Eleanor (Jane Alexander). This is quite problematic for the film because their relationship just does not work. I do not think that this is Jones' fault but rather Alexander's who is downright dull and uninteresting. She does nothing to add to the depiction of the relationship, and although Jones tries to show a genuine relationship, Alexander makes any effort on his part entirely wasted.
Jack's boxing wins and relationship with Eleanor gets him into trouble with the authorities, causing him to have to go to Europe, and subside off things like arm wrestling, and putting on a deamining show of Uncle Tom's cabin. Jones is good in showing the slow degradation of the spirits of Jack as he is forced to lower and lower things, eventually being given an offer to throw a fight against a white opponent. Jones is effective in showing his bitterness building up, and slowly losing his his trademark smile that he once had.
Jones's is effective in the role I think, despite the film holding a lot against him. Alexander makes it so the pivotal relationship amounts to nothing, and frankly many of the scenes could have allowed a more interesting performance if it was not so exceedingly stagy. Overall though his performance I think is good, but I think it could have been better with a better co-star and in a better film about the same subject.