Ten years ago, when I noticed there was a lot of whispering in Mad Men, someone always pulling another aside in the midst of a chaotic office to ask if so and so had heard the news about such and such, I brought this up with another writer who asked, incisively, how one might show whispering in fiction.
I've been trying to pay special attention to whispering in fiction in the years since but have come across surprisingly few instances. One might think, given the intimate nature of interpersonal conflict in realistic fiction, we'd overhear whispering characters more often. Fiction writers, though, have many ways -- primarily the manipulation of setting -- to get characters alone and have them speak in confidence, which is why, I suppose, we don't often come across characters speaking to one another in fear of being overheard.
That being said, suppose you did want information secretly shared between characters in a crowded setting. The most obvious way is to simply state that one character is speaking softly, doesn't want to be heard, and so on. We this in Jane Austen's Emma, when Franck Churchill says to Emma "This is a pleasure." Austen tags the dialogue with "in a low voice." Later, Emma will continue the conversation "in a whisper."However, most style guides and craft discussions suggest it's best to avoid tagging dialog in this way. They reason that tagging dialogue is less elegant and artful than conveying the feeling of what is said by words alone. The effect of tagging, or reporting how something is said, doesn't, typically, allow a reader to feel how something is said.
Generally, this isn't bad advice. Yet the scene in Emma comes off more artful than general fiction writing advice might predict. But this is because Austen doesn't merely tag her dialogue. She takes care to set the scene, not only crowding the room with people, but also adding music, as Jane Farifax is playing on the piano. The crowd and noise, working as a contrast, bring out the whispering, making it all the more apparent. The scene is also remarkable as a parody of whispering and speaking in confidence, since it ends with everyone in the room overhearing Mrs. Bates yelling through the window to Mr. Knightly.
Before coming across this scene, I'd tried to think about solving the problem of representing whispering by imagining how you might show its opposite, yelling. Capital letters have this effect, though most stylists would look for more artful means. The exclamation point is another way, and Austen uses many of them when Miss Bates is at the window. Nonetheless, thinking in terms typography didn't get me very far.
Another memorable scene in which one character speaks to another in whispers comes from Andrew Milward's short story "Skywriting." The main character, Stephen, a recovering drug addict, has confessed to stealing from his mother's friends. He falls to his knees, cries. She accuses him of being on drugs. When he stands up, "she pulled me close and whispered so softly that her breath on my and ear felt like a kiss."
Although Milward uses the word whisper he withholds what Stephen's mother has actually said. On the one hand, it's classic suspense. Something important has happened but we don't yet know what. Additionally, it has much the same effect as witnessing in real life two people talking quietly across the room. Further still, he characterizes the gesture as gentle, touching, comforting, "like a kiss." This is a subtle, strategic move, since it's not what she's said that feels like a kiss but her breath. We won't learn for a few paragraphs, the very last line of the story, in fact, what she's said. I'll leave that for you to discover on your own.
