Pose of Silence

A character wants to talk about another behind his back, but is in the same room with them. He assumes the pose of silence. He leans forward in a conspiratorial manner, rendering everything he says perfectly inaudible to the other, even if only a few feet away in an otherwise silent room.

Stage Whispers, where the asides and "private" discussions are held in full voice (loud enough for the audience to hear, obviously) but other characters freeze or act as if they hadn't heard it, are an established dramatic convention in live theatre. A similar convention surrounds soliloquies, where one character makes a speech to the audience but is unheard by any other characters on stage. These conventions are Older Than Steam, dating back at least to Shakespeare.

A Room Shuffle can be arranged, to dodge this trope and provide a little action.

Film

 * In the film version of Harry Potter and the Chamber of Secrets, Harry, Ron and Hermione discuss their secret plan which will "break about fifty school rules" while walking down a corridor crowded with students. (In the book, they discussed it in private.)
 * Played for Laughs (or something like it is Played for Laughs) in Monty Python and the Holy Grail, when the French knight is bawling over the castle wall to Arthur and his knights that they already have their own Holy Grail, then turns to the other knights with him on the wall and whispers behind his hand, "I told them we've already got one!" and they all cover their mouths and guffaw, clearly learning this for the first time.

Literature

 * A lampshade is hung on this when C-3PO leans over in an "overly conspiratorial" fashion in a Star Wars Expanded Universe novel.

Live Action TV

 * The most blatant examples of this are from Friends, where it would seem the characters are all deaf, and an invisible soundproof wall was found just behind all sofas.
 * And menus when the characters being discussed are sitting at the same table...
 * Happened all the time in Married... with Children, typically when the family was mocking Al in front of his back, so to speak.
 * Given that it's Al being talked about, he might still be able to hear them. They just don't care.
 * Much like Shakespeare's characters, Malcolm in Malcolm in the Middle performed this regularly for the audience. This was played with once onstage, while Frankie was presenting for an awards show.
 * The Animorphs TV series had the heroes insanely hold most of their meetings in a crowded arcade/restaurant, where anyone around them could be one of the bad guys. In the books, they logically meet in the privacy of a barn.
 * They did this in the books, too, albeit wit less sensitive information.
 * Used repeatedly on on The West Wing. On Ainsley's first day, she has to meet with two Jerkasses about a stupid thing they did, and suggests that they go out into the hallway rather than discuss it in their open-plan office. Being new, she doesn't realize that the hallway will be as full of people walking and talking as any given room. She leans in and says, "Okay, we'll keep our voices down." One Jerkass leans in long enough to sarcastically say, "Okay," then straightens up and they both spend the rest of the scene in normal poses talking at normal volumes, while she continues to lean and whisper. Can't get no respect, can't get no Pose of Silence.
 * Averted a couple of times in Hannah Montana, and even receiving of a lampshade hanging or two.

Western Animation

 * The trope was also played with in the "Tales From The Public Domain" episode of The Simpsons. It contained a Hamlet reenactment, in which Moe butted in on Bart's soliloquy.
 * Once more, in an episode of The Fairly OddParents. Crocker, who had been invited (or, rather, invited himself) over for dinner, used the trope along with his own characteristic hunch to plan out his schemes and insult those around him, only to find them standing right next to him and able to "hear single every word [he's] saying." The bit was then, in the show's oh-so-typical manner, repeated two more times.