Earpiece Conversation

A person is coached through a conversation of some kind by someone else who feeds them the right lines through an earpiece.

This will inevitably lead to a situation where the prompter is distracted by something and says or does something not meant to be heard and repeated, but the person on the other end faithfully mimics it anyway. The Idiot Ball is promptly passed to the person wearing the earpiece as they do as they were instructed, never considering even for a second that screaming out "OH GOD! NOT THE FACE!" might not be the best thing to do while you're trying to impress your dream date in a crowded restaurant.

Sometimes a variant occurs where the receiver's connection goes bad, and what would otherwise be the perfect thing to say gets misheard as something else.

Often overlaps with Playing Cyrano. If the prompter manages to talk to his buddy on the airwaves AND the people around him at the same time, it's a Multitasked Conversation.

Examples of Earpiece Conversation include:

Advertising

There is one Priceline commercial featuring William Shatner advising a husband speaking to his wife on travel deals. Taken literally by the husband, who starts speaking like Shatner.
A Canadian credit union has an ad where a young girl applies for their unlimited savings account featuring unlimited withdrawals, deposits, transfers, inquiries, and sugar cubes in the bank's coffee. The teller asks if she's using this trope when she begins grilling him, and her dad says "Tell him 'no'" through her earpiece.

Films - Live Action

In the movie Roxanne (basically a remake of Cyrano De Bergerac), Charlie (Steve Martin) talks to Roxanne (Daryl Hannah) through the ditzy Chris, who is wearing a goofy hat to hide the earpiece. They're using a Fire Department radio, so it ends up like this:

Charlie: (coaching Chris) Reach out your hand.
Chris: My hand, out reaching to - car three, car three! Proceed to the 279.
Roxanne: What?

Sneakers has a goofy example of this. When Martin gets surprised by the mistress of the man whose office he's breaking into, his backup team starts feeding him plausible excuses to use on her for his being there. And then some less plausible stuff...

Whistler: And give him head whenever he wants.
Martin: And give him he... help. Be a beacon in his sad and lonely life.

Primer has a plot-central instance of this, where Aaron is being instructed on what to say by recordings made by himself in alternate timelines, in an attempt to keep divergence between the iterations to a minimum. It doesn't work.
A variant occurs twice in Dave, with the title character mistaking his handlers' stage directions for the next line he should say. "Go, go go." "And Go! Go! Go!"

Live-Action TV

In Battlestar Galactica, Gaius Baltar is often being instructed in this manner by Virtual Six.
There's an episode of Radio Active wherein Kevin is being coached through an oral exam by a hidden microphone with Morgan on the other end.
In Coupling, Jeff is being talked through a possibly-a-date with a co-worker, which ends up in a Multitasked Conversation.
Another Playing Cyrano example from Will and Grace, where Jack was hitting on a man while working in a retail store where he wore an earpiece. Will feeds him lines so he can look smart. The guy isn't taken in, finds Will, and asks him out.
The D Generation did a sketch of them attempting to feed answers to jane Kennedy during her appearance on the celebrity edition of Sale of the Century.
On one episode of Muppets Tonight with guest star Cindy Crawford, mumbling security guard Bobo the Bear develops a crush on her. They hook him up an earpiece so that Rizzo the Rat can feed him the lines, but Rizzo's nephews (coming home from somewhere with balloons) walk in, leading Bobo to hear and repeat: "Hey that's a nice set of balloons, mind if I play with them?"
In Drake and Josh, this is done by having Josh feed information to Drake through an earpiece in a Quiz Bowl-esque contest to impress a smart girl.
Since the Leverage gang literally wear earpieces all the time, this happens periodically. For example, Sophie is pretending to work for an African consortium that wants to buy a bunch of airplanes from the mark of the week:

Sophie: I represent a group of investors who are looking to start an airline for short tour flights in Africa.
Dubenich: Out of Johannesburg?
Nate: Okay, he’s testing you. You want Bloemfontein.
Sophie: Keep away from the hubs. Revitalize the regional airports. In South Africa Bloemfontein for example. But really it’s Nigeria we’re focused on.
Nate: Yeah, perfect.

- See "The Stork Job" for one of many incidents of this leading to hilarity:

The Mark: Where are you from?
Nate: Pittsburgh, Detroit, Atlanta
Parker: Pittsburgh, Detroit, Atlanta.
Nate: No, pick one, Parker.

Western Animation

6Teen: Catelyn did this once.
On The Simpsons there was a gag where Kent Brockman was being fed lines this way even when he was socialising.
In the Pinky and The Brain episode "The Pink Candidate," Brain coaches Pinky through his presidential campaign speeches this way. While Pinky is holding a press conference, the wireless earpiece fails and starts picking up other random transmissions (a bank intercom, a drive-thru, a sports announcer), but by coincidence the answers still please everybody.
In SpongeBob SquarePants Patrick coaches SpongeBob through his boating test by a walkie-talkie hidden under a cowboy hat. Mrs. Puff compliments SpongeBob on his improvement, joking that it's as if someone were telling him all the answers. But when she mentions that that would be cheating, SpongeBob freaks out and confesses... and crashes the boat, failing the test once again.
In the Lilo & Stitch: The Series episode "Spike", Lilo discovers that the reason for her rival Myrtle always getting the answers correct at a quiz show was because one of the members of her posse had been giving her the answers through an earpiece that she had been wearing the whole time, which is cheating. Fortunately, Lilo sends Experiment 613 aka "Spike" to lower that girl's IQ, causing Myrtle to lose the second round to Lilo.