The protagonist is at a crucial turning point in their life and things are generally looking pretty grim. Rather suddenly, the character is surrounded with bags and bags of letters. While the letters are written by people who aren't even in the story, the sheer quantity of envelopes implies that they accurately represent public opinion, constitute valid legal arguments, or create some other opportunity for the character they are addressed to.
Sometimes, for instance in Miracle on 34th Street, the letters aren't even read or opened. Other times, for instance in Mr. Smith Goes to Washington a character will read a few aloud and the audience is left to assume that all the letters sound like that.
The mass of letters often appear on camera in literal bags although many movies use the trope to refer to letters offscreen or just show a pile of letters which were presumably in a bag at some earlier point.
This trope is a form of Deus ex Machina since most of the letters are written by characters who aren't even in the story. The effect is usually positive, but the letters can occasionally have a negative effect on the protagonist.
Variations due to advancing technology include having someone rush into the room saying "The Switchboard is Lighting Up" where it's been established that the character is on live radio or television and is about to be kicked off the air when suddenly hundreds of people start calling in, as well as (briefly in the '90s and early '00s) a rapidly filling email inbox or In-Universe real-time reaction via social media such as Facebook, Twitter, or the equivalent.
- In Swing Vote, the drunken hick holding the future of the country in his hands, receives a number of these that he is too lazy to check. As a result, his daughter does her best to answer them, just to make the people sending them feel that someone's listening. At the end of the movie, the aforementioned hick uses the letters to question the politicians, thus cutting through the B.S. and getting their real opinions, as well as responsibly representing the American people.
- In Miracle on 34th Street, at the end of the famous courtroom scene, bags and bags of letters addressed to Santa Claus are brought into the courtroom until they are piled high enough to block out the judge. While the prosecutor argues a few letters delivered by the USPS don't constitute a valid legal defense, the judge decides the bags and bags of letters do constitute a valid legal argument.
- In Mr. Smith Goes to Washington, Mr. Smith expects his filibuster to sway public opinion in his favor, but he is presented with bags of letters that reveal public opinion has turned against him. The mass of letters almost makes Smith lose hope.
- In The Upright Citizens Brigade, they lampoon this trope. A guy says there's a truck full of letters supporting his cause, but they're just random letters like bills and coupons. And he just insists that there's a whole truck outside. And he's accused of having stolen a mailtruck to support his cause.
- In It Could Happen to You, after a journalist (who masqueraded as a homeless man, to see how they would react: despite the fact they were penniless, they helped him out) runs the story about this poor couple, thousands of people send in contributions and raise $600,000 for the couple to live happily ever after.
- The title of the movie Letters from Iwo Jima refers to a bag of letters from Japanese soldiers on the island that were never delivered to their families. Their discovery by modern day archeologists starts off the story.
- In Sleepless in Seattle, after announcing how hard it is dealing with the death of his wife, the protagonist receives thousands of letters from around the country and one of them is from the girl who he will eventually fall in love with.
- In Harry Potter and the Goblet of Fire, people Rita Skeeter attacks in her Daily Prophet articles, such as Hagrid, Hermione and Harry himself, tend to receive large amounts of hate mail. Although it's not shown, Dumbledore claims that he was also sent a lot of letters vouching for Hagrid (from people who knew him from their school days). This is reversed in the fifth book when Skeeter is blackmailed into conducting an unbiased interview with Harry (also an aversion of this trope, since the resulting letters vary from "You're a hero!" to "Seek professional help").
- In Adorkable, when Jeane wants to give up her blog, believing that no one really cares about it, Michael makes her read some of the E-Mails she has received - loads and loads of E-Mails. She changes her opinion.
- On The Dick Van Dyke Show when Sally said on national TV that she was looking for a man Bags of Letters arrived overnight.
- On The Late Show with David Letterman the Viewer Mail Theme Song (borrowed from The Perry Como Show) goes "Letters / We get letters / We get stacks and stacks of letters / LETTERS"
- In the Hancock's Half Hour episode "The Bowmans" Hancock finds himself bombarded by mail the day after his radio Soap Opera character is killed off.
- In a sketch on Saturday Night Live the two biggest stars of South American music release a joint album while at the same time taking potshots at each other through song. In a duet to the tune of "Guantanamera" one singer claims he gets one ton of fan mail. The other singer counters with the fact that he gets two tons of fanmail. The first comes back by alleging that he gets four tons of fanmail. It then devolves into claims of eye jobs and trouser stuffing.
- Variation: The original Bill Cullen edition of The Price Is Right utilized randomly selected postcards in five rotating drums for their home viewer sweepstakes. Whoever had the exact price first out of five selected cards (or the first closest to the price without going over) was the winner.
- In "Message in a Bottle" by The Police, the message gets 100 billion replies.
- Frequently parodied in I'm Sorry I Haven't a Clue, with lines like "I see from the small mountain in our in-tray that someone's sent us a scale model of Kilimanjaro" or (in August) "I see from the vast pile of envelopes in front of me that Christmas delivery is as good as ever", leading to the announcement that they've only received one letter, from a Mrs Trellis of North Wales.
- At the end of Both Your Houses a corrupt congressman's office is getting swamped with telegrams opposing a pork- and graft-laden appropriations bill.
- In The Solid Gold Cadillac, Laura Partridge finally defeats the Corrupt Corporate Executives by adding up letters from small shareholders granting her voting control over their stock.
- In Julius Caesar, Cassius forges letters, pretending to be Roman citizens, to persuade Brutus to take action against Caesar for the good of Rome. Achieves the same effect as honestly written letters.
- The Simpsons:
- When they own a racehorse, as he's being put out to stud the horse gets Bags of Letters from prospective broodmares.
- When Homer files a lawsuit against a seafood restaurant whose definition of "all you can eat" differed from his, the defendant asked for a display of how much Homer ate that night. At that point parodying the film Miracle on 34th Street, a huge convoy of people carrying large sacks enter the court, but it turns out that they just contain letters for an adjacent courtroom (People of Springfield vs Santa Claus, IIRC).
- Celebrities will frequently refer to "bags of letters" talking about fan mail or viewer feedback.
- Robert Englund got so much fanmail when he played Willy on V (1983) that they eventually started to use unopened fan letters as props in other programs. In his biography, he mentions seeing a studio tour where the guide pointed to a stuffed letterbox and said, "All of those are letters to Willy."
- Craig Shergold was diagnosed with a terminal illness. He made it known that he wished to break the world record for most postcards received before he died. He got better, and soon was up to his neck in cards.
- While she was working on Star Trek: The Original Series, Nichelle Nichols was told by a couple of studio mailmen that she was receiving bags of fan mail, but that they were being dumped — apparently for racial reasons. It got her so miffed that she wanted to leave the show and only Dr. Martin Luther King Jr. was able to talk her out of it.
- According to DJ Simon Dee of Buccaneer Broadcaster Radio Caroline, when the first tender arrived after the station's 1964 opening, what he thought were sacks of potatoes on its deck turned out to be sacks of mail.