In the novella's dialogue, the reader notices subtle differences in the language of various characters. Although everyone speaks English (other than occasional French or Italian phrases), James carefully individualizes various characters' dialects. This difference in language further accentuates the difference in character between the American expatriates and the Millers. Although they speak the same language and even come from the same country, their self-expression varies substantially.
Relative to the formal and polished language of the Europeanized Americans, the Millers' language tends to be more casual and frank. James distinguishes the characters' dialects through vocabulary, grammar, syntax, and tone. Additionally, there is a difference in volume. Whereas Winterbourne is concise and brief, Daisy's dialogue tends to take a substantial amount of space on the page.
In terms of vocabulary, the Millers make frequent use of fillers and interjections like "well" and "you bet." They also make steady use of qualifiers like "so," "very," "really," and a number of other adverbs. When it comes to grammar, the Millers occasionally contract "does not" to "don't" rather than to "doesn't"—a convention the reader would never expect from characters like Winterbourne, Mrs. Walker, and Mrs. Costello. James emphasizes the Millers' unique tone of voice through exclamation points and italicized words. All of these characteristics contribute to the casual American sound of the Millers' speech.
James makes these differences apparent in the novella's first conversation. When Randolph approaches Winterbourne in the beginning of the story's action, he bluntly asks him for a piece of sugar "in a sharp, hard little voice." Later, when he tries to bite down on the sugar, he exclaims "Oh, blazes, it's har-r-d!" The narrator notes that he pronounces the adjective "in a peculiar manner." Randolph's speech doesn't only stand out because he's a child. James uses him to show the reader what an American raised in America sounds like, relative to a cosmopolitan American.
The characters themselves notice the differences in their dialects. For example, the Millers find Winterbourne's dialect ambiguous. Soon after meeting him, Randolph asks Winterbourne if he's an American man, which indicates that his accent doesn't make this immediately obvious. This sense is reinforced when Daisy asks Winterbourne whether he's a "real American" and states that he seems "more like a German" when he speaks. In the other direction, the Millers' dialect leaves Winterbourne with no doubt about their provenance. However, interacting with Daisy does make him realize that he has become "dishabituated to the American tone."
Already in the novella's first pages, the characters' differences in dialect make it difficult for them to read each other. These linguistic differences are increasingly accentuated by cultural and behavioral differences. James puts careful effort into making the Millers' speech casual and uncultivated while ensuring that the Europeanized Americans speak in a polished, reticent way.