Brad Pitt plays a solid Jesse James: frightening, off kilter, somewhat mentally unbalanced, and terrified of losing his power. His strategy is to keep those under him unsure of what he will do next. He thrives on having men around him who fear him. He keeps Robert Ford around him longer than others because Ford fears him and also idol worships him.
Casey Affleck as Robert Ford is amazing. Casey is Ben's younger brother and usually plays the village idiot. His first role I recall is playing the underling to Matt Damon and Ben Affleck in Good Will Hunting. And he reprises this type of character as one of the two getaway drivers in the Ocean's 11, 12, and 13 movies. In this film, although he still plays someone's doormat, he is underestimated by those around him.
At the beginning, we see the last big hurrah of the James brothers. The older brother seems to sense that his retirement is well advised at this time. After a disaster of a robbery (Jesse hesitates before killing a man), the older brother decides to live out a law abiding life. Jesse feels somewhat abandoned until he spots Robert Ford, who begs to be allowed to join his band. Jesse doesn't exactly take a liking to the kid, but he seems to need him for this time. He senses Ford's obsession with him with a patient, but distant, disdain.
Several mini-plots come up, which are all well done, but the film is really about Ford's transition from almost in love with James to becoming his murderer. The transition is slow but complete. Ford plays a Judas type killer. He knows it has to be done, but he's guilty about it.
I found the last part of the film to be the most intriguing. After the death, Ford experiences a brief time of being a hero. He is photographed by the dead corpse. He is even asked to repeat the incident on stage in a play. Although the law had been after Jesse James for years, he was an icon. You will have to decide for yourself why Ford becomes one of the most loathed characters in all of American history. Was it that fact that he killed James in a coward's way by shooting him in the back? Was it because America considered James to be a hero and not a villain? Was it because James was smooth, suave and handsome and Ford was awkward and clumsy? I found myself feeling sorry for Ford.