Sometimes, characters will answer questions from readers (though sometimes the questions are only meant to seem as if they came from readers, such as in Family Guy), as if the Fourth Wall had a mail slot. This is particularly common with Webcomics, since the nature of the medium makes it easy to set up multiple pathways for audience feedback, such as a Shout Box, a blog-poweredNews Post, or a Message Board.
One reason that webcomickers have given for answering questions in-character is to not give as many spoilers, since then the answers are limited to what the character knows. The Fourth Wall Mail Slot is also a handy gimmick for sketching quick Filler Strips to prevent Schedule Slip, though many webcomickers keep it separate, as a bonus extra, many (such as El Goonish Shive) include it as a regular end-of-issue (or story-arc) feature. Some don't even make the in-character Q&A into a comic, leaving it as plain text instead, or with just a small illustration. Occasionally this is used to reveal All There in the Manual-type information about the storyline that hasn't been mentioned in the comic yet.
Naturally, given all these advantages, it's not surprising that webcomic artists will sometimes blatantly make up stuff to push through the slot. ...wait.
See also No Fourth Wall, Post Modernism, Character Blog, Official Fan-Submitted Content, Letters to the Editor.
The manga version of Bleach has "Radio Kon", in which Kon and various guests answer reader mail. In her appearance, Tatsuki isn't aware of the whole Fourth Wall business, and wonders why people are sending in these personal questions.
Loveless: In "Good Morning Loveless-kun", Ritsuka and Soubi answer "fan mail" and try to find the mystery of the lady...
Magical Girl Lyrical Nanoha had a few included in magazines that feature it, where cast members would field common fan questions. The Megami Sound Stages has various cast members that appear in the series answering, and an Animedia article had Hayate doing it on her own.
D.Gray-Man actually had a hilarious crossover bit with this, when a question intended for their Fourth Wall Mail Slot wound up in the inbox of Gintama creator Hideaki Sorachi... who then attempted to answer the question semi-seriously. You can read about it on this Livejournal.
The Gintama anime has the Ginpachi-sensei segment where letters (usually about Fridge Logic) get jokingly answered. On one memorable occasion, a supposed kid sent in their version of the show's pseudo DVD covers.
A regular feature at the closing of every Nana volume. The characters even show knowledge of Manga of other publishers.
Sket Dance doesn't have a regularFourth Wall Mail Slot (even if it does sometimes Break The Fourth Wall in various ways), however when a character popularity poll for this manga was held, the results were presented in one of its chapters, showing the reactions of various characters to the results, and particularly the protagonist's constant anxiety about which place will he get.
Blue Exorcist has various characters answering questions at the end of each volume, first happening at the end of volume 8.
Fairy Tail has Mirajane and Lucy usually answering questions at the end of each volume.
Sayonara, Zetsubou-Sensei has an instance where the characters take a poll on what the subject of the author's next manga should be. There's also a time when Chiri and Jun answer reader complaints in a Dear Negative Reader manner.
This was hardly invented by webcomics. One of the earliest examples: the Fantastic Four used this trope many times in the early years, even making the mailman into a humorous stock character.
This was revived in Jonathan Hickman's run, where Franklin and Valeria would respond to reader letters.
Marvel Comics "superhero" Deadpool has been answering his own letter page for years. Then again, the comic and the character himself are both severely lacking in the fourth wall department.
The British version of the comics has a letters column run by Soundwave, and later Grimlock. (Who would often angrily try to explain away his defeats from previous issues.)
The British comic based on the film series does this as well. At first, it was run by Starscream (who wasn't pleased when readers sent him mail about how much they liked the Autobots). It was later taken over by Ironhide and Barricade, who had a lot of fun with continuity issues and Barricade's unexplained disappearance. Near the end of 2011, when the comic was retooled to focus on Transformers Prime, the letters page came to be run by that show's incarnations of Arcee and Megatron.
DC Comics had several examples of books where the characters, not the editor, answered the mail:
Legion of Super-Heroes vol. 4 took this to the next level. Its occasional attempts to bring back the lettercol are done in comic book format, with the Legionnaires reading out the letters and discussing them.
When Guy Gardner had his own series, he answered his own letters.
Ambush Bug answered his own letters, and in his first issue was puzzled that he had to wait months for feedback from the reader who had just finished the book.
Deadpool isn't the only Marvel character who has answered his own fan mail either; in the early 1980s, the letter column of the Uncanny X-Men comic was answered by a designated character each issue, resulting in things like Wolverine claiming that Rogue dyed—well, bleached—her trademark white streak into her hair.
This has recently been brought back in the letters page of New Exiles. Since they're based outside the multiverse, it was actually possible in-story that they might be receiving letters from a dimension where they're fictional characters. They thought it was weird, though.
In the Marvel UK children's title Spectacular Spider-Man Adventures, responses to readers' letters were presented as if they were written by Spider-Man.
Letters to the The Sensational She-Hulk were answered by the title character or one of her supporting cast.
In Sonic the Comic, responses to readers' published letters were initially presented using the persona of the comic's mascot character Megadroid, and in later issues the responses were presented as coming from Sonic the Hedgehog.
The Italian Official Nintendo Magazine one time did it with a Boo, a Lakitu, Professor Oak and Tingle.
In the Cerebus letters pages, a regular correspondent mentioned Cerebus throwing a baby off a roof. In a later issue, Astoria berates Cerebus for that action. He replies to her (and to the letter writer), pointing out that he threw the baby from the front steps; he threw an old man off the roof. Next issue had a letter from the original correspondent feeling crunchy for having been corrected by Cerebus himself.
The letters page of British Anthology Comic2000 AD is answered by the editor. The editor happens to be Tharg the Mighty, a green-skinned alien from Betelgeuse on a mission to bring Thrill Power to the planet Earth. Some have claimed that he is fictional, seeing as other comics of the era often had fictional "editors" or "hosts", but this is clearly false.
The current Hellcat miniseries has a letters page known as Hellpcat, which is more like an advice column than a traditional letters page. But Patsy Walker still answers it in character.
Vertigo Comics' Fables series did it one notch better. They ran a contest for the most interesting questions, and in issue #59 "Burning Questions" they answered them by making each of the answers a fully-canon vignette.
Even Archie Comics has done it, with advice columns answered by "Betty and Veronica" in their 1970's and 1980's issues. Josie had a column as well. Later on, the columns were mocked with a Reggie version, where he took letters meant for Betty and Veronica and answered them sarcastically. The latter is possibly the funniest thing Archie has ever printed. It's in Archie and Friends #5, if you want to hunt it down.
Bill & Ted's Excellent Comic Book had a different character answering the letters for each issue. It got particularly surreal in issue #8, when the character answering the letters was Station, who can only say his own name. The answers in that particular letter column consisted of Station replying "Station!" to all the letters, with some unhelpful attempted translations by the comic's editors, who spent more time arguing over how much sense it made to have an entire species with a one-word vocabulary than actually translating Station's replies.
Doonesbury does the "reader mail" bit periodically. Lately it's been relegated to an annual thing, usually happening the first week of the year. It's a bit of an aversion in that the questions are usually about the production of the comic itself and not the storyline. Also, while the letters are real, the answers are usually blatantly false (like the time someone asked why the strip had no Hispanic characters, and Mike replied with a list of prominent recurring Hispanic cast members...none of whom had ever been seen before.)
British weekly comics have a long history of this. In the mid-1960s, TV Century 21 had a letter column that was answered by "S", the head of the Universal Secret Service that was featured in one of the strips in the comic. Later, the job was taken over by Colonel White of Spectrum when the Captain Scarlet and the Mysterons strip began.
Jok Church's U Can strip is all about the mail slot, asking kids to send science-based questions to Beakman and Jax, the sciency sibling protagonists. This proved so popular that a television series was made out of it.
Ask Shagg is entirely based on this trope, with a Ridiculously Cute Critter answering children's questions about the animal world.
Dragon Ball Abridged has released two FAQ specials where the characters answer fan questions. They occasionally insult the fourth wall as well.
Chiaotzu: "Can I be a part of Team Four Star?" Oh yeah, sure. when you grow some freakin' talent. Tien:' Chiaotzu! That was totally uncalled for. Now they'll just waste their lives in a futile struggle to measure up to their peers until they're nothing more than a crumple heap of countless failures and broken dreams. Right, Yamcha?
Some fans on Tumblr create their own askblogs, answering questions in character, in a mix of roleplay and fanfic writing, with their replies usually drawn or written.
The Next Frontier uses a variation, with questions and comments from readers being answered in-universe as if they were actual comments on Jeb's blog, the framing device for most of the exposition.
Films — Live-Action
In the Christmas special, Santa Claus Is Coming to Town, you would have the narrator reading mail that have questions for Santa. Questions that kids would really ask. However, the audience would hear the kids' voices and not the narrator's. During the movie, the voices would randomly appear sometimes as if they were with the audience.
One interpretation of Michael Haneke's Cache/Hidden is this trope.
The website of the Alex Verus series has a semi-regular question and answer column called Ask Luna, run by Alex's apprentice.
David Letterman does a mail segment once a week — "Viewer Mail" on Late Night, "CBS Mailbag" on Late Show.
David Letterman (shaking a handful of blue cards): Ladies and Gentleman...if these weren't actual letters from actual viewers .... could I do this?
Mystery Science Theater 3000 frequently answered viewer mail, oftentimes, due to the nature of the show, from kids. It was quite often adorable, but just as often it clearly grated on the host's nerves (especially Mike).
The entire point of the show Mail Call. R. Lee Ermey (Gunnery Sergeant Hartman in Full Metal Jacket) answered viewer-submitted questions about military equipment, often consulting with experts and throwing in a dose of drill-instructor haranguing for comic effect.
Planet's Funniest Animals has a segment called "Animail", where the host reads viewer mail that is so ludicrous or specific that is has to be fictional. He then answers by showing a clip related to the mail entry.
Done (with fictional letters) in one episode of Teen Angel.
Lizzie McGuire had commercials in which actors would answer emails fans sent, in character.
Red Dwarf's Smeg Ups special featured Kryten answering the 10 (allegedly) most common questions from fan letters. He answered the questions increasingly quickly, (one was "what are the lyrics to the closing theme?", which he answered by reciting the lyrics faster than Busta Rhymes) and became increasingly frustrated with rhetorical questions relating to continuity errors, which accounted for about half the questions ("BECAUSE WE MADE A MISTAKE!")
Top Secret! had a couple of character originally created by an editor-in-chief who had to make an entire issue by himself and tried to cover up the lack of personnel. They later began appearing in comics featured in the magazine, but also kept occasionally "writing" articles and were pretty indistinguishable from real editors (it didn't help that one of them was the editor-in-chief's Author Avatar). As a result, they frequently ended up answering letters (or having letters addressed directly to them), and some readers couldn't tell they were not real people, wondering who is hiding under these nicknames.
In the long-since-ended Adventurers Club magazine (a house organ for Hero Games), the earlier letter columns are "written" by lunatic supervillain Foxbat. After Foxbat's apparent death in the artwork for Champions II, a different character took over the job of answering the mail... but it wasn't as funny.
Paizo's message boards have messageboard threads where players can ask questions of the various  Iconics, and they will often answer.
When playing NetHack on some Unix systems, if the player happened to receive a new email while playing, a mail daemon would appear in-game and deliver a "scroll of mail" to the player's character with the email.
BlazBlue: Calamity Trigger has unlockable comedy scenes in Story Mode called "Teach Me! Ms. Litchi!" where Litchi teaches Taokaka how to be a vigilante. This segment is unrelated to the storyline, and it doesn't give gameplay tips, but it does explain how the world of Blazblue works.
Arby 'n' the Chief has one that was introduced in the fourth season. Now it has become it's own series as a spin-off to Arby 'n' the Chief.
Hentai parody website Zone-Archive includes ZTV NEWS episodes to announce upcoming events on the site; they start with the "Zone-Archive Mailbag", where mascot Zone-tan reads questions from the subscribers. (Link SFW, and the ZTV NEWS videos themselves are visually SFW, though they do contain crude language.)
Freefall has a thread on one of its two forum pages called "Q and A with the Freefall Cast and Crew" here. The creator, Mark Stanley, responds as the character who was questioned.
Achewood's typically subversive version of this was an actual problem page where readers mailed in their genuine sexual and personal problems. This was always accompanied by the disclaimer that, since the advice was being dispensed by Ray (a cartoon cat who, even if he were real, would probably not be the best person to take advice from) it would probably be best for the correspondents to not make any life decisions based on his answers.
The "Ask Kyo" section of Inhuman, though it is a separate guestbook instead of an actual side comic.
Every so often, Something Positive will have Choo-Choo Bear, the lovable boneless cat of Davan, answering fan questions, usually in an insulting way, while he lords his obvious superiority over us insignificant beings (and especially the worthless hack who writes him). Sometimes giving cut-shots to exactly how he (and/or the author) feels about some of the weirder questions — most infamously "What if the cast are furries" panel.
Adventurers! has a few filler strips that feature the comic's author answering questions from fans.
Honeydew Syndrome has characters answer readers' questions in the extras section, though only the guys from chapter 1-5 have been subjected to this so far.
Nodwick subverted this; although the characters have answered mail on two occasions, it referred to the characters rather than the author, indicating presumably that they were made up for a joke rather than being actual fanmail.
Errant Story has the occasional "Errant Commentary" pseudo-filler strips which feature Sara and Bani answering questions, lampshading everything, and abusing the fourth wall.
Keychain of Creation also features this, though it's managed by The Fair Folk, who in-setting are thoroughly aware that "all the world's a stage", and in-comic become antagonists because if they weren't in the comic, they wouldn't exist. (We are not making this up.) The humans don't seem to understand where the Fourth Wall mail comes from.
Axe Cop has Ask Axe Cop, a side feature where readers ask Axe Cop questions. Not only is it just as funny as the main storyline, it is also a must-read, since some concepts and characters, like the T-Rex Wexter, get introduced first here and later put into the main comic without explanation of where they came from.
Black Tapestries does this a few times. Funny because most of them are furries complaining about Lorelei not liking her werefox form, and Lorelei yelling back at them.
Rachel from Dragon City does this weekly in a segment called "Ask Professor Rachel" in which she answers questions about the creator, the creator's other comics, dragon society, dragon biology, and so on. While she sometimes makes comments about what's going on in the story, in the comic/outside of Ask Professor Rachel, she does not retain this knowledge. She also cannot break the fourth wall inside the comic unless someone else does and she's scolding them for doing so. She also makes no indication inside the comic of having any knowledge of Ask Professor Rachel.
A Loonatic's Tale has one at the end of every main-story print edition. For the omnibus edition of the first three issues it was answered by the Cruor brothers, while Talking To Myself instead features Dr. Robert Chester and a surprise visit from Dr. Wilhelm Qubert's hitherto-unknown spiteful side.
Lackadaisy's artist occasionally answers reader questions in bonus comics.
Subverted in Exterminatus Now, when the gang gets an email telling them to stop slacking off, and go do some work for the Inquisition. Eastwood gets in a tiny rant before Virus points out that the e-mail was from High Command.
Bittersweet Candy Bowl recently started a tumblr blog where reader questions are responded to with watercoloured comic panels, it can be found here
Chapter 50 and 58 of Welcome To The Convenience Store concentrated on answering the most commonly asked questions (the manager commented that the artist/writer was just being lazy).
Jet Dream And Her T Girl Counterspies is a parody of comic books circa 1970, and includes parody letter columns in which letters are frequently addressed to, and answered by, the characters themselves.
End Of Infinity allows readers to send Telegrams to the characters, and any non-narrating character is forced to obey them. THE POWER OF TELEGRAM COMPELS YOU!
The Star Trek official website — when they had an official website, that is — had a number of "in character" mailbags, including two at one point running concurrently, answered by a Klingon and a Ferengi, who would at times take swipes at each other in their columns! Then they got rid of them and went with a refugee from that gangster planet from the original series. It wasn't as funny. The one-shot answered by Captain Pike, however was pure comedy gold.
Friends of Foamy has multiple episodes featuring this, with Foamy getting more and more irritable with each one. They even allow Pillsy and Germaine to sit in for one of them.
The BioShock 2 viral website There's Something in the Sea includes a working PO Box number for the (unseen) protagonist, Mark Meltzer. Fans who have tried writing in have received copies of the drawings in the puzzle box. Most of the letters appearing directly above Mark's filing cabinet are, in fact, written by fans. They have also received stuff like membership forms for the "International Order of the Pawns" and a coded message by Lutwidge that took the cooperation of more than a dozen people to decipher.
In the early days of the lonelygirl15 series, "Bree" would respond to the users' comments at YouTube.
Used in the Personifications subfandom — after all, it is technically just a collection of ask blogs.
Used at the end of one of the episodes of Brawl Taunts.
The Lizzie Bennet Diaries had 100 episodes plus 10 "Questions and Answers" sessions. Lizzie asked her viewers to send her questions through Twitter, Facebook or Youtube, and those that were interesting for the story or characters got answered in-universe. In addition, the creative team was always eager to communicate with fans at the official page or various social networks.
The Autobiography of Jane Eyre is a Setting Update vlog inspired by The Lizzie Bennet Diaries. Jane communicates with her viewers through commenting at the video, on her Tumbrl page or Twitter, and she also does Q&A sessions. Jane's actress Allyson Hall and others from the creative team sometimes offer their views and comments as well, either directly at the videos or on their social network pages.
A number of cartoon blocks in the 90's (mostly for Fox and WB) had "contests" or something of the sort every so often where the characters from shows in the block would read and answer letters from fans in tandem with footage. More often than not, these would hire the actual voice actors to read them.
Cartoon Network did this, in that they had an address where you could write in to a favorite character and they would either read it on air, or they would reply via snail mail with a postcard. Shows like Cartoon Planet and JBVO also had addresses to which viewers could send letters.
In the kids' science show The Magic School Bus, there is a "Producer Says" segment at the end of each episode, in which a fictional producer of the show receives fictional phone calls from fictional kids complaining about how some things that happened on the (fictional) show couldn't happen in real life. This was a way of defusing the more fantastic aspects of the show so as to keep to its educational agenda.
VeggieTales used to use this quite often, as a framing tale for the moral-tastic shorts.
Parodied in CatDog where a fan mail segment was done on the very first episode. As expected, they are told that they didn't receive any fan mail yet forcing them to answer Dog's question. At the end, however, they discover that Winslow hid all the fan mail from them.
Ask Vector Prime was continued in the Transformers Animated AllSpark Almanac II, using questions invented by the book's writers. Unfortunately for readers, the questions were in Cybertronix and needed to be translated.
Parodied on The Ren & Stimpy Show as "Ask Doctor Stupid". The comic book would also have an "Ask Doctor Stupid" section for responses to real letters at the end.
Done right in The Ripping Friends. Craig was usually the one who read the mail, with a sock puppet bunny getting beat up or something. The mail usually had a problem happening in the mailer's life, so the Ripping Friends would go and fix it by tearing it apart with their bare hands.
In the middle of its run, Toon Disney had a page on their site where viewers could write messages to Timon & Pumbaa, Lloyd, Rope Girl, and Tino, and get an automated response. Amazingly, they kept this on the site even after the shows and the channel itself disappeared.
Parodied on KaBlam! on a few occasions. One had a request for Henry to "put his hand over his face". He proceeded, then June karate-chopped him in the face, and told the audience that she sent that request in herself.
Clerks: The Animated Series did this twice in its six-episode run; once in the third episode Dante and Randall answered letters from the audience, which introduced their black friend Lando, and in the final episode the boys hosted a panel at a convention and took questions from the audience. The nature of the questions inspired Dante to make the series more like the original movie, leading to the series's second Locked in a Room episode.
Parodied on Dave the Barbarian. At the end of the episode, the audience is tricked into think that they're getting some answers when they see Dave in a director's chair "on set".
Dave: You're probably wondering why I tied a squirrel to a megaphone. (beat) Well, goodbye!