Like all good dramatists, O'Neill shows some of the characters' different traits through their different use of language. Mary tends to be very proper, almost awkwardly so. She shies away from crude remarks and refuses to acknowledge any reference to her morphine addiction or to Edmund's illness. On the other end of the spectrum, Jamie will say whatever comes to mind, however insensitive or inappropriate, such as the comment that gets a punch from Edmund: "Where's the hophead?" (4.1.88).
The difference between Mary and James's diction may have something to do with their age (the younger generation in the play is freer with language), but James and Edmund sort of straddle the line between polite and rude. James will say things like "You're a fine lunkhead!" (1.1.101), but mainly when someone says those taboo words "morphine" or "consumption."
At the beginning of the play, Edmund's almost as afraid of confrontation as Mary is. After realizing that the family is suspicious of her, Mary asks Edmund why he's concerned, giving Edmund the first opportunity the family's had to broach the morphine subject. Instead, he backs down, answering "evasively": "Nothing. Just because I feel rotten and blue, I suppose" (1.1.201). By the end, though, he's ready to call Jamie a "dirty bastard" (4.1.89).
Direct characterization's really the money characterization tool here. It's pretty much what O'Neill does best. The stage directions give us all the tools we need to understand these characters, detailing their actions, clothes, physical appearances, and habits.
O'Neill tells us exactly what's going on and, often, what we should make of it. For instance, he doesn't just say, "Mary's hands are ugly and rheumatic." He says:
Her hands are never still. They were once beautiful hands, with long, tapering fingers, but rheumatism has knotted the joints and warped the fingers, so that now they have an ugly crippled look. One avoids looking at them, the more so because one is conscious she is sensitive about their appearance and humiliated by her inability to control the nervousness which draws attention to them. (1.1.stage directions)
Notice how O'Neill uses physical description as a way of getting at a character's psychology? We're not sure where else in literature you could find an author who sketches the personalities of his characters so directly.