Midge Kelly at the beginning of the film is shown to be the champion of the world in boxing. It then flashes back to his very humble beginnings as just a normal Joe traveling through with his brother Connie Kelly (Arthur Kennedy). They are both struggling to find his way, and early here Douglas is his usual very charming self. Douglas early shows Micheal a charming guy, who is quick to get into a fight to make a quick buck, or defend himself or his brother.
Midge and his brother find themselves on the wrong end of a scam, and instead of owning part of a diner they end up workers at it. There is the small concession of the beautiful daughter of the diner owner Emma (Ruth Roman), who Midge charms quickly, which is quite natural since Douglas is especially charming here. Midge and Connie leave though after Midge is forced into a shotgun wedding by Emma's father once he finds out that she and Midge are having a relationship. Midge instantly leaves with Connie after the wedding, Douglas here suggests well another side of Midge, a darker side that refuses to be controlled, or kept in a position by someone else.
Midge finds he is a natural in boxing, and after training quickly begins winning matches. Midge stays charming for awhile, and passionately refuses again to throw a fight to appease gamblers. Because of this and his fairly sudden rise to the top this brings out a different side of Midge. Midge who forgets his old manger who helped him from the beginning and his brother. Douglas becomes properly cold, and loses his charm. Douglas suggested this of Midge early on and he slowly realizes this change in Midge for it to be properly effective.
Midge begins not only to disregard everyone else for his success but he also begins to coldly manipulate them as well. Douglas is coldly effective in two scenes particular, where he manipulates a woman Grace Diamond he has being going with for awhile who attempts to manipulate him, but his cold denial of her, and his rejection of her is outstanding. Another manipulation Midge does is he manipulates his wife not to get a divorce. Douglas shows Midge to still be charismatic, but now in a stark cruel fashion. This change of Midge is chilling because Douglas properly brought Midge to this point, and also because the degradation of Midge as a person is so great.
Douglas' performance is especially effective because he does not try to portray Midge as a villain but as a truly fallen man. He did not fall exactly because Midge was always an bad man, but Douglas portrays it that he did because his want of survival, and his refusal to be looked at as lower than someone else. I particularly think Douglas' final moments are incredibly powerful because he shows the real honest cause of Midge's fall at the end of the film as he spouts himself as the Champion and finally showing everyone else that he is better well being punch drunk in a sad final moment of a once charming honest man who has fallen to nothing.