Michael of III is not the Michael of I and II. By III he's less taciturn, less introverted. He repeatedly ignores the first rule of, "Never let anyone know what you're thinking," no matter what he tells Vincent.

I cannot resolve that Michael III is Michael of I and II and that's a defect in the story. We needed to see Michael as cold and dark and introverted as at the end of II and then watch how he tries to make amends and fails. Michael III is sober but casual, considerate but emotional. He's lost his gravity and the only way I can see that happening is if he's forgiven himself his past crimes. He's too resolved by the time III opens. We cannot analyze Michael beyond I and II because Michael III isn't of the same creation or history. He's not a different man because of age, he's different because he was written, badly, that way. It's not Pacino's failing, he worked with what he had.

Ignoring III, I see Michael descending further into paranoia and depression.

Michael's the youngest, obviously his father's favorite, and his father has very high expectations for him. Vito places his own failed desires on Michael, using Michael to achieve the legitimate life he could not have. Michael is perceptive, intelligent, and handsome; the golden child of his family. Though his father indulges him in joining the Marines, Michael is acutely aware of his father's desires and seeks to earn that love by being exactly what his father wants. Conversely, when Vito is shot, Michael becomes the instrument of his father's revenge, falling out of the legitimate life to do the honorable thing-- as Vito would have. Vito impressed upon Michael two things he even tells Michael directly, but has obviously impressed upon Michael before: the importance of family and the importance of accepting responsibility (men cannot be careless). In the end Vito is disappointed with Michael and Michael, feeling spurned, takes on Sonny's role of proving himself to his father for the rest of his life.

Sonny is bossy, cocky, tempermental, and strives to be his father's favorite. Sonny is out to prove himself and his power through sex and violence but still cannot gain the favor of his father.

Fredo is the family wastral. He lives on the periphery of the family. Likely his mother's favorite, Fredo sinks his disappointments into adolescent desires. Considered slow by his family, they don't expect much from Fredo and indulge his rebellious nature. Fredo obliges using his freedom as a compensation for the lack of love and respect he sought from his father. Even when acting as bodyguard to his father, Fredo is absolved of responsibility when his father is shot. The one time Fredo steps out of his role proves fatal.

Tom, though son in name, plays the dutiful guest. Tom seeks to repay his father by joining the business and being loyal to all his brothers. Tom's sense of duty is so engrained that it overrides all moral sense. In turn, Tom's brothers resent Tom's presence. Sonny feels Tom doesn't do enough and Michael suspects Tom of disloyalty. This causes Tom to continue to work to please his brothers, even submitting to the assassination of Fredo, out of fear of losing the only family he has.

The women of the family are an odd mix. Michael was right to reject Kay, knowing that Kay was too educated and independent to make a suitable wife for the life he's chosen. Appollonia was a much better choice; used to living in a town saturated with mafiosi. Like Mama Corleone, she would have kept her head down, willfully pretending she's unaware of what her husband does but not so unaware that she questions things, as Kay does, or doesn't ask her husband for special favors from time to time as Mama Corleone does. Kay is a modern American woman and for Michael, she represents the legitimate life he forsook to avenge his father and protect his family. The dichotomy of the relationship proves untenable, Kay knows she's being lied to, lives in a lonely gilded cage, and resents it, while Michael sees her as the living embodiment of his failure to be legitimate and resents her for being what he should have been but cannot. He says this on their walk after his return from Sicily. He will become worthy of her, become her equal, be free to share openly with her as he did before (at Connie's wedding), but in the end he's forced to (literally) close the door on Kay. The first time because he cannot be what he promised her, the second because he believes Kay has betrayed him and the family. He spares her from death because at least some part of Michael knows the marriage failed largely because of him and so she is not truly to blame.

Of all the characters in III, Kay and Connie become the most credible. Early on we see Connie hating Michael for killing her husband then going out to do her version of the Fredo thing. Never independent, she was raised in the old country tradition of being a wife and mother yet, when robbed of those by her own brother she forsakes being a mother to instead become a wastral, perhaps hoping that her mother will somehow intervene and punish Michael. When this doesn't happen, Connie becomes Michael's surrogate wife, managing his home, raising their kids, and becoming Michael's confidant, his consigliari de familia. Her anger turns inward on herself, seeing Michael go unpunished for his crimes, she comes to believe it was she who was wrong in marrying Carlo, it was good for Michael, "to be strong for all of us," even if it meant killing family members. By the end of III, Connie is a black widow, emulating Michael by swearing deadly revenge on Michael's enemies no matter the cost. She consoles herself in celibate bitterness derived from a Stockholm syndrome-like love for the man who killed her husband and brother and the incestuous frustration of being unable to consumate that love with her brother. She is light years from the happy bride we saw at the beginning and the most direct result of Michael's twisted attempts to mould the family in his father's image.