As you read Ulysses, you'll notice that Joyce uses about every possible tool of characterization there is except for direct explanation. If we get an explanation of a character, it is always filtered through the voice of someone else in the book and so we have to question it. There are numerous instances of a character's actions defining them. The most obvious example is Blazes Boylan. The entire character of Boylan comes from the fact that Boylan is the guy who is going to have sex with Molly. This one fact dominates everything else we learn about him, and instantly makes us dislike him. In the case of Stephen and Leopold, however, they are characterized not so much by their actions as by their passivity. Stephen announces to Private Carr that he has actually made a philosophy of passivity, and Bloom decidedly does not prevent Molly's affair. Their passivity frustrates out expectations for main characters in a novel, and forces us to re-asses both of them and to think about the difference between action and passivity, and the relation between thought and action.
Clothing is often detailed meticulously in the story. From the very start, we learn that Stephen is borrowing clothes from Mulligan and that his own are extremely threadbare. Boylan, by contrast, dresses quite fancily and we get an image of him as a dandy – think Algernon in The Importance of Being Earnest. The largest role that clothing plays in Ulysses is in the dress of both Leopold and Stephen. Stephen is wearing black in mourning for his mother, and Bloom is wearing black in mourning for Dignam. As they move about town, people often comment to one another about their dress and how morbid it is. We usually associate goodness with light, but here we are given two dark men as the heroes of our story. The idea of darkness and its associations with passivity, uncertainty, anti-heroism, and many other ideas, plays a central role in the story.
The very first lines that introduce Leopold Bloom reveal him as a creature of habit: "Mr Leopold Bloom ate with relish the inner organs of beasts and fowls. He liked thick giblet soup, nutty gizzards, a stuffed roast heart, liver sliced fried with crustcrumbs, fried hencod's roes. Most of all he liked grilled mutton kidneys which gave to his palate a fine tang of faintly scented urine" (4.1). Perhaps the major contrast in habit is between Stephen's spartan lifestyle and his otherworldly pre-occupations, and Bloom's closeness to nature and the earth. Bloom is, for example, a man of tremendous appetite. Though he cannot have sex with women, we also see that he loves women and can't help himself from checking out young girls' bottoms as he moves about town. The characters are, quite realistically, creatures of habit, but as the novel moves on we begin to sense how they might be constrained by their habits and feel the need to break free of them. Stephen's tendency toward solitude, for example, has become extremely problematic for him.
In case you were wondering, "Dedalus" isn't the most common Irish name in Dublin. It is a reference to the father of Icarus in Greek mythology, who built wax wings for him and his son. Icarus, though, flies too high – getting close the sun, which melts his wings – and falls back to earth. Stephen, son of Simon Dedalus, is toys with the heights to which his artistic ambition might take him, but also beginning to learn just how far he can fall. Similarly, "Bloom" can be read as having a number of meanings. Leopold's pseudonym is Henry Flower, which he thinks is quite clever. There are numerous punning references to his name, but perhaps the dominant idea is that, though Bloom is middle-aged, his life is still unfolding – blooming. Many of the other names in the novel actually come from real Dublin characters, but Joyce simply loved words too much not to have fun with names when he could.
Stephen is a teacher in the novel, but he is mainly defined by his idleness, by the fact that he is unemployed. Much of Stephen's wandering is a search not so much for an occupation as a "vocation" – finding a task that he thinks is worthy to his talents. Bloom is an advertising man at the newspaper. For the record, a lot of hyper-literary people have a prejudice against advertising men because they are wasting any creative talent they might have trying to get people to buy a product that they probably don't need anyway. Advertising is one of the many disadvantages Bloom starts off with at the opening of the novel, but further in we begin to see advertising in a more full light. At one point, the repetition in advertising is compared to the repetition in Church – where you are trying to beat a message into someone's brains. Molly and Boylan also seem to be somewhat defined by their occupations. They are both singers, and thus are expected to me emotional and passionate types, which Molly, at least, proves to be.
None of the major characters in Ulysses is particularly well-off, but social stature in town seems to be extremely important to them. Stephen's father Simon was at one time a reasonably wealthy man, but has since fallen into ruin. We see this in graphic detail in "The Wandering Rocks" when we get a peek at the impoverished home of the Dedalus's. Yet, you'll notice that Simon is still well-respected about town. Stephen is something of a cast-about, and though he finds his element in "Aeolus" impressing people at the newspaper office, we find that others in town are not so crooning. In "Scylla and Charybdis," Eglinton and Moore unintentionally snub Stephen by not inviting him to a reading, and we hear Simon complain of the friends that Stephen keeps. Part of the problem simply seems to be that he sets himself off from everyone else in Dublin. Lastly, Bloom has constantly been in and out of jobs, but right now seems to have a degree of stability. Yet his social acceptance is not linked exactly with his status. He is half-Jewish, tends to preach when the other men are joking, and doesn't drink heavily. All of these things make Bloom an outsider in Dublin society.
What is particularly interesting about Ulysses is that there is an enormous difference between what people think and what they say, and we get to see this discrepancy at work because we are privy to their thoughts. There are a number of famous quotes from the novel. Stephen, for example, gets associated with "History is a nightmare from which I am trying to awake" (1.157). Bloom gets associated with "Force, hatred, history, all that. That's not life for men and women, insult and hatred. And everybody knows that it's the very opposite of that that is really life" (12.423). Both characters are, in part, defined by these lines and their willingness to voice them, but we can't rush to the dialogue to get a full idea of the characters in Ulysses. Their thoughts are extremely modulated and nuanced, and because much of the novel takes place in stream-of-consciousness, we have to follow these statements into their innermost reflections in order to get a complete picture of them.
Joyce's stream-of-consciousness style was revolutionary. In Ulysses, we get a new version of interior monologue in which the characters don't think in complete and well-formed sentences. Instead, their thoughts are fragmentary, disjointed, often turning from one idea to another on the simple basis of wordplay. The result is that their thoughts are also often inconsistent and contradictory. Yet we do get a general feel for the character by forcing ourselves to follow the twists and turns of their stream-of-consciousness. Bloom, for example, has an creative mind, and often thinks of ordinary things in humorous and off-kilter ways without even realizing that he is doing so. Stephen's thoughts in "Proteus" prove to be erudite and sophisticated; they are focused more on the spiritual realm than on the earthly. Molly's long interior monologue at the end of the novel reveals her to be an extremely passionate and freethinking woman. Whereas Stephen's and Bloom's thoughts are often clipped, starting and stopping, for Molly, thinking flows.