Tools of Characterization
Characterization in The Godfather
Actions
Since these characters are all mainly gangsters, they're characterized by the criminal stuff they do. Probably the best examples would be when Michael shoots Sollozzo and Captain McCluskey, and when he organizes the massive slaughter at the end of the movie. Both of these actions show that his transformation into a mob boss involves more than just a change in attitude—he has bloody deeds to back it up.
Michael begins as a good kid—a war hero from WWII, an Ivy League student. But that all changes when he decides to kill Sollozzo and McCluskey. He demonstrates that he has the steely will to kill without compunction, when he deems it necessary.
Yet there was evidence for this from earlier in the movie. When he lights Enzo's cigarette after they pretend to be armed men and scare away would-be assassins from attacking the injured Don, his hand isn't shaking (unlike Enzo's). He has the coolness, the classic Hemingway-style "grace under pressure" to handle tough situations.
Unfortunately, he ends up using these skills in the service of murder and evil deeds.
Clothing
A lot of these gangsters are pretty well-dressed. They're wearing fine suits. But Sonny seems to dress fairly casually (in wife-beaters, for instance), illustrating his scruffiness and the fact that he's not really put-together.
By contrast, Don Corleone is pretty dapper, and Michael has a more business-oriented kind of fashion sense. Sonny mocks his high class-style when they discuss avenging the Don's near-murder:
SONNY: You've gotta get up close like this and—bada-bing!—you blow their brains all over your nice Ivy League suit.
Oh, and at the beginning of the movie, Michael wears his Marine Corps uniform to his sister's wedding—which helps mark him apart. It shows that he's (ostensibly) different from the rest of his family (at first), and also suggests that he might possess a military kind of personal discipline.
Family Life
You have positive and negative examples of family life in The Godfather. Carlo viciously beating Connie would be a definitively negative one. But the way Don Corleone interacts with his children is pretty genuine—he really cares for them.
It's this kind of bond that, while on the surface seems like a nice thing, ultimately forces Michael into pursuing a life of crime. Loyalty to the family means adopting a life of violence. "Family values" actually becomes the villain, in a weird way.
Family is extremely important to the Corleones. Even a little divergence from family loyalty is a major problem. Fredo tries to argue in Moe Greene's favor after Michael threatens to buy him out. But Michael gives him a sharp response, illustrating this attitude memorably:
MICHAEL: Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the Family again. Ever.
Food
Food is a reminder of the Corleones' Italian culture, which helps define them as outsiders in their society. It's an aspect of who they are. At one point, Clemenza explains how he cooks tomato sauce to Michael:
CLEMENZA: Heh, come over here, kid, learn something. You never know, you might have to cook for twenty guys someday. You start with a little bit of oil. Then you fry some garlic. Then you throw in some tomatoes, tomato paste, you fry it. Ya make sure it doesn't stick. You get it to a boil. You shove in all your sausage and your meatballs, heh? And a little bit o' wine. An' a little bit o' sugar. And that's my trick.
Location
The movie is set mainly in two important locations, one in the New World and one in the Old: New York and Sicily. New York was a major center of mafia activity in America, being home to the famous and deadly Five Families. Sicily, on the other hand, was the original home of this version of organized crime, the place where it all started.
Michael has to travel to Sicily to escape being charged with the murders of Sollozzo and Captain McCluskey, but he's also getting back to his roots. He probably ends up feeling more of a connection with the Old World's traditions, which solidifies his determination to work for his family's success.
On the other hand, New York (and Las Vegas, where Michael briefly journeys) represents the changing flux of the modern world—the world of drug-dealing and high-stakes casino battles—to which the Corleones need to adapt. Michael becomes familiar with the Old World while trying to bring some of its traditions intact into the New World.
Names
Most of these characters are Italian-American, and have Italian surnames. But certain characters like Tom Hagen and Kay Adams are marked apart by their different ethnicities (Tom Hagen is Irish-German by descent, and Kay's last name is probably English in origin).
Their names shows that, even though they're both close with the Corleones, there are limits to how much of a role they can play in that world. Tom Hagen can act as a trusted adviser to the Don and be his adopted son, but he can't actually become the Don. Kay can marry Michael, but she can't endorse or participate in his criminal life.
Also, Sollozzo's nickname, "The Turk," says something about him. But it's not a reference to his ethnicity: He has drug connections with the poppy-growing business in Turkey (heroin and other opiates are made from poppies). He signifies the new wave of the future, the transition of mafia families from gambling and prostitution and racketeering to drug-dealing.
Occupation
The fact that many of the guys in this movie are gangsters obviously says a lot about them. They're not exactly in line with the morals and conventions of the wider society. Even though Don Corleone doesn't regret living his life this way, he wanted something better for his son:
VITO: But I always thought that when it was your time, that you would be the one to hold the strings. Senator Corleone, Governor Corleone, something.
Michael, however, seems increasingly suited to his occupation. He takes to it with zeal. In his eyes, it ends up being a path all the way up the ladder. He figures that he can kill all his enemies and fight his way to the top. "Legitimacy" is just a word to him.
Physical Appearances
Brando's version of Don Corleone has a sort of "bulldog" appearance: He looks tough, but he also has a kind of warmth and humanity. Sonny looks scruffy, and also sort of wild and animalistic. Michael, on the other hand, has a cooler, sleeker, and more unruffled appearance.
Social Status
The Mafiosi in The Godfather are in an odd position: They're rich and they have power, but they're a little too controversial to really be high society. They view themselves as being on the outside, trying to assert themselves in a world that isn't entirely friendly.
For instance, Bonasera is a law-abiding citizen until the courts fail to adequately punish the men who attacked his daughter. He complains to the Don:
BONASERA: I went to the police, like a good American. These two boys were brought to trial. The judge sentenced them to three years in prison, suspended sentence. Suspended sentence! They went free that very day.
The subtext here is that Bonasera wasn't able to get justice because he's an Italian immigrant, and 1940s American society looks down on him.
Thoughts and Opinions
One of the most common opinions expressed by the mobsters in The Godfather is that the things they're doing aren't personal, they're just "business." For instance, when Tom Hagen counsels an irate Sonny, he says, "Even the shooting of your father was business, not personal, Sonny!" But Sonny's never able to see this. In his world, everything's personal.
But Michael is able to say, "It's not personal, it's strictly business" and mean it. This attitude is what makes him such an effective criminal, but it's also what makes him an ultimately reprehensible person.
He's able to order Moe Greene's murder without compunction, for instance, and order a hit that doesn't just kill a mafia Don but the woman he's in bed with, too. These things are, basically, evil—but since we see Michael's path and his philosophy, it's a little more comprehensible to us. It's not the kind of inexplicable evil that, say, Hannibal Lecter from Silence of the Lambs represents.