In fiction, it doesn't matter how quietly you speak, people in the other room can still hear you. For example, if Alice and Bob are in the kitchen discussing plans for Aerith's birthday gift, with hushed voices, then Aerith, who's living in the apartment across the hall will mention she doesn't want the gift they were talking about.
In most cases, there is an ongoing secret that is central to the character or the plot, and the hero or villain is talking freely about it with the Secret-Keeper or the one who Cannot Spit It Out. Unknown to them, someone else got near and overheard it. The time for The Reveal has come! Don't forget to tune in next episode, same hour, same TV channel! And, in the next episode, everything is solved with a contrived explanation. "Yes, I said that I would kill Storm, but I was not talking about the X-woman, I was just talking about the weather.".
Another possible outcome is that the character actually heard the hidden truth, but comically missed the point and understood it all wrong. For example, The Ditz overhears a communist conspiracy, where someone points out that when their Evil Plan is ready, the Party will find anyone. But instead of realizing the threat, she understands that they are fun guys, with a project so that anyone will find a party. Of course, the evil ones will be worried when they realize that she heard them, and breathe easy when they hear the misunderstanding, and try to maintain the ditz in the misunderstanding (for example, hosting a party or two, so that the thing stays credible).
It is possible as well that the character heard well, understood what he heard and the significance of it, but can't or won't take a megaphone and tell everyone. Perhaps the secret holder has a way to force him into secrecy, such as blackmail. Perhaps the character will put a price on his silence. Perhaps the secret sounds so unlikely that even if he talked nobody would believe him, not without a Smoking Gun. Perhaps he just doesn't care. Or perhaps he was Killed to Uphold the Masquerade. In any case, it's bad news: the more people that know the secret (even if they don't reveal it), the more likely that it will become an Open Secret and reach ears it's not supposed to. Even death is not enough: unless it was a Faceless Goon, if a character is killed someone will investigate who killed him and why. And that leads to even more people lurking around the secret!
- In various commercials for Canadian Club Dry, one guy asks his presumed drinking buddy "what's wrong", to which he replies "I don't like beer, anymore". The other patrons hear this and go quiet, even though he whispered his response and they were talking fairly loudly.
- The Life and Times of Scrooge McDuck: In The Raider of the Copper Hill, Everyone in town somehow overhears Howard saying to Scrooge that he needs to put the claim on his land to get rich.
- In Supergirl storyline Many Happy Returns, Dr. Faulkner takes Linda Danvers aside to discuss Kara's biology and mental state. All of sudden Kara speaks up and tells she can hear them whispering in the other room clearly. Justified because she has Super Hearing.
- The Girl with the X-Ray Mind: Shortly after giving Lex Luthor powers and convincing him to ally himself with them, the Phantom Zone criminals huddle together aside to argue how they will betray him later... clearly forgetting Luthor now has super-hearing, and zero reason to not eavesdrop. After sabotaging their ploy, Lex calls them out on their stupidity.
- Ultimate Marvel:
- Ultimate Daredevil & Elektra: Melissa and Phoebe set a trap for Trey, gloat about it once he's no longer in sight... and failed to notice that they did so right in front of Matt.
- Ultimate Spider-Man:
- Flash and Kong discuss in the library that there is a bounty on a photo of Spider-Man. And yes, Peter was there.
- The enforcers are talking about the Kingpin. Ox and Dan want to turn against him for petty reasons, Ox tries to force some sense into them. And the clarifies "Don't even joke. You never know who's listening". Spider-Man is hidding just above them.
- Short Circuit has the head scientist 20 meters away from the bureaucrat and calls him an idiot.
"I heard that!"
- In Office Space, right as Peter, Michael, and Samir hatch their scheme to rob Innitech and swear each other to secrecy, Lawrence, Peter's next door neighbor, calls through the wall that he won't tell anybody their plan either. Michael and Samir begin to freak out over the fact that they were overheard, but Peter assures them that Lawrence "is cool." That Lawrence can hear everything that goes on in Peter's place (due to the thin walls between their apartments) is something of a running gag throughout the movie.
- A justified example in I Heard That Song Before. When she was six, Kay sneaked into the old chapel in the Carrington mansion her father had mentioned. She heard two people coming in and hid between the pews to avoid getting into trouble, and thus overheard a man and woman arguing about money. When the woman promises this will be "the last time" she asks for money, the man sarcastically remarks "I heard that song before", then leaves the chapel whistling the song of the same name. Neither of them realized Kay was there and she never mentioned it until two decades later, as she didn't realize it had any significance.
- One of the basic key elements of soap operas and telenovelas, it happens Once per Episode and frequently even more. Basically all plots involve some form of Easily Overheard Conversation, and the big secret is overhead several times before the final and open reveal.
- Happens from time to time on Our Miss Brooks. Usually, the eavesdropper misunderstands and hilarity ensues.
- A running gag on Benson: whenever Benson says something negative about Kraus, no matter how quietly he says it, or how far away Kraus is, she will yell out, "I heard that!"
- In Buffy the Vampire Slayer, Dawn continually overhears conversations about her in season 5. Despite the problems this creates, they never learn.
- On The Big Bang Theory, it's called Sheldon's "Vulcan Hearing". One example is when Raj whispers into Howard's ear about Sheldon, and they both laugh. Then Sheldon walks back into the room and announces that he heard them. Cue guilty looks and Howard muttering, "Damn his Vulcan hearing."
- Usually lampshaded in Graduados by Andy, who is always complaining that her parents always get behind the door to overhear his private conversations.
- There is a Love Triangle in Solamente Vos: Félix, Aurora and Michelle. Felix is talking with Michelle in the phone, and suddenly Aurora comes very serious: she's tired of being "the other", she's tired of his constant promises of breaking up with Michelle that never became true, she's tired of not having a real romantic life but just the few spare moments he gets from his relation with Michelle... and, while she talked, Felix forgot to close the phone: Michelle heard it all.
- Usually a Justified Trope in Supernatural as angels and many other supernatural creatures have Super Senses.
- On New Girl, Cece is in Jess' room talking with her about getting breast reduction surgery. Suddenly from outside the door comes a Big "NO!" from Schmidt, who after a long beat (implying that he was listening from across the hallway) comes bursting through the door.
- War of the Worlds (2019): Sacha is able to overhear a pretty shocking revelation from his mother and uncle's conversation when in the bathroom while they speak nearby.
- The Assassin's Creed series has the player eavesdrop on people who are sometimes up to 30 or 40 feet away, often on crowded streets. This may be part of the assassins' heightened senses, along with Eagle Vision.
- Invoked in Metal Gear Solid. Snake listens in on Liquid and Ocelot talking about the override for Metal Gear REX, saying that only the PAL key can deactivate it. Of course, when Snake uses the key it activates REX, which was their plan all along.
- Because the contestants on Total Drama are on camera, pretty much everything they say is "easily overheard" to some degree. The most trope-relevant instance is when the campers forget this whilst bonding at the bonfire after the Episode 6 elimination, thereby giving Chris all the information he needs to set up the phobia challenge.