Throughout Lady Windermere's Fan, the tone mostly remains witty. A significant portion of the dialogue consists of clever, but perhaps equally unaware, characters exchanging quippy one-liners and silly remarks. At the same time, Wilde balances his comedy with gravity. A few of the main characters resist the silly banter of their friends and acquaintances, and even some of the wittiest characters give the audience occasional access to their vulnerable sides.
By the end of the second act, it seems clear why the Windermeres are such a good match. Surrounded in their social scene by quick-witted characters who eagerly display their epigrammatic cynicism at every turn, Lady and Lord Windermere always remain serious and sincere. This immunity to the attitudes of those around them is apparent in their lines.
For example, in Lady Windermere's conversation with Lord Darlington in the first act, she not only insists on being taken seriously, she begs him to speak in a forthright way. This is partly because she doesn't want him to pay her inappropriate attention, but also because she prefers earnest conversation. When he laughs at things she says, she tells him that she is "talking very seriously," that she is "quite serious," and that he "mustn't laugh." Lord Darlington reveals to her that the last thing he wants is for the world to take him seriously. Because Lady Windermere is the odd one out, her tone ultimately feels melodramatic. Even in her more serious moments, it's hard for the audience to take her completely seriously.
By contrast, it is when the lines of the characters who never seem serious at all take a serious turn that the play's tone truly undergoes a chill. An example of this is when Mrs. Erlynne explains to Lord Windermere that the "terrible" suffering she experienced the night prior showed her that she can never embrace the role of mother. Not only does the content of this speech touch the audience, the stage directions indicate the character's radical shift in tone as she delivers the speech: "Mrs. Erlynne looks at him, and her voice and manner become serious. In her accents as she talks there is a note of deep tragedy. For a moment she reveals herself." Halfway through the speech, the stage directions indicate that she is "hiding her feelings with a trivial laugh."
The initial conversation between Lady Windermere and Lord Darlington introduces the tension that will remain throughout the play between its witty tone and its occasionally serious, even melodramatic, tone. The majority of the characters contribute to the play's lighthearted tone with their bold and euphemistic remarks. It is when these characters, who only seem capable of surface-level banter, reveal themselves on a deep and vulnerable level that the audience feels the most moved.