Tex Avery likely codified this trope, as one of its first appearances was in the Red Hot Riding Hood short, where the actors complained that the story had been done to death by every animator in Hollywood.
Cartoon Network used to play a meta version of this trope in their commercials, suggesting that all the toons actually worked in their offices.
The last one actually had a person asking "Are you supposed to be the actual characters, or the actors playing them?" to which the answer was "Absolutely."
In I Am Weasel, the director was the devilish series antagonist.
Sheep in the Big City did this constantly. They would often cut to the narrator as he commented on a particularly ridiculous aspect of the storyline. One episode takes place "behind the scenes", with General Specific attempting a Hostile Show Takeover.
A variant is the oft-copied Looney Tunes short Duck Amuck, in which Daffy Duck has a running argument with a malevolent animator who repaints the scenery and Daffy himself to twist the reality of the story. Warner Brothers even did this again, to Bugs Bunny, in the short Rabbit Rampage. This was further repeated when the latter short was remade into a Super Nintendo game which gleefully referenced the original short plus several other Bugs Bunny cartoons, with the final boss being against Daffy Duck in some of his appearances (including Duck Dodgers, Robin Hood Daffy and Drip-Along Daffy). And again when the original cartoon was remade into a Nintendo DS game. In a Twist Ending, the player abusing Daffy is Daffy himself.
Other Looney Tunes cartoons played with the idea that the film was actually a stage show, allowing Bugs to turn and address the audience (who in some cases talked back, an effect that was lost on youngsters watching them on television), and treating the screen as nothing more than a backdrop on that notional stage. Porky Pig, Elmer Fudd, and several other Looney Tunes characters have either threatened to resign from or torn up their acting contracts during a cartoon if things begin to go badly for them.
Taz has done this at least once in Taz-Mania.
The Scarlet Pumpernickel has Daffy Duck complaining to an unseen producer about being typecast in comedy and pitching his script for a swashbuckler starring himself.
In the Duck Dodgers series, the opening credits treat Dodgers, the Cadet and Commander X-2 as characters played by Daffy, Porky and Marvin.
Clerks played with this a lot in the opening sequences, for instance having characters answering fan mail (well, mostly hate mail) and attending a convention.
As usual, The Simpsons subverted it in an episode where Homer goes to an art gallery which is displaying Matt Groening's work. He says something like "Matt Groening! He doesn't belong in a museum! He can barely even draw!" Then a giant pencil eraser appears and seems to "erase" Homer. We then zoom out to see that the giant pencil is being carried by two museum workers and is part of the scene. Another episode was a mock "behind-the-scenes" episode, featuring the supposed "actors" that portray the characters, with (for example) the actor playing Lisa complaining that they made her take anti-growth hormones to keep her as a child throughout the show's run.
In The Critic, Jay Sherman addresses the audience a few times. One episode he does this is "A Little Deb Will Do Ya," in which he tells the audience not to reveal the said episode's twist to those who haven't seen it.
The first episode of Tiny Toon Adventures began with this trope, and the artist drawing Buster, Babs and the rest of the Toons for the first time.
Another episode reverses the typical "big character is actually a guy in a suit" gag by having Sweetie, a tiny pink bird who is probably the smallest regular cast member, turn out to actually be Richard Nixon, who is three times bigger than the costume he's wearing. After zipping the costume back up, Nixon (in Sweetie's voice) complains that he's only getting paid scale.
VeggieTales uses this trope in just about every video/episode. There are the kitchen countertop scenes with Bob, Larry, and maybe another character like Junior, and then they break away for the story. The same "Veggie Tales" characters will play multiple roles in all stories and silly songs. For example, Archibald Asparagus is voiced by Phil Vischer, but Archibald plays the part of Jonah in The Movie.
The American Dad! episode "Bullocks to Stan" featured the character Klaus commenting on scenes, explaining that he was pretending his life was a TV show and he was doing the commentary. This even happened in one scene that Klaus did not appear on, and explained that the extra playing a chef was the same man who had played somebody else in a previous scene, saying that the man who had been intended to play the chef ("Jimmy Ng") had died during filming. The credits for the episode played over an animated scene of the various "actors" from the show in a behind-the-scenes moment, hugging an unseen character, with the caption "Dedicated to the memory of Jimmy Ng".
An episode had the action stop and pull out to reveal a set and a crowd of people gave Roger an award for American Dad's '1000th vagina joke'.
An episode had Stan getting fed up with the uninteresting B Story he was in and walking off the set while complaining about the writing. Earlier in the same episode, Steve fell out of a window and had to say his line while in casts.
At the end of "She Swill Survive," we meet Stan and Hayley's actors: Nicholas Vanderbilt and Kate Fagan.
In the episode "Candle Jack", Freakazoid! uncased himself from a rope he'd been tied up with to get up and thank the director and his co-stars. There is a slight difference in that, for instance, he refers to the fact that Cosgrove is voiced by Ed Asner. At the end of the series finale, Freakazoid again thanks the cast and crew, bringing them out for a curtain call and leading them in a stirring rendition of "We'll Meet Again"
In another episode a large number of bit characters who had not been seen in some time showed up, demanding to know why. Freakazoid told them that, due to financial problems, the network couldn't keep them all in any position above washing his Freak-Mobile.
The planned Grand Finale of The Angry Beavers, "Bye Bye Beavers", demolished the fourth wall by having the characters openly discuss their impending cancellation and complaining about how the network would continue to make money off them while they lost their jobs. Naturally, the network wouldn't allow this to air and production was halted before animation started. (There is, however, a recording of the voice work done for this episode floating around the internet.)
The Magic School Bus, when originally shown on PBS, included an ending segment where the show's "producer" (an animated character voiced by Malcolm-Jamal Warner) answered questions supposedly phoned in by kids who had just watched the episode. Some episodes even featured characters from the show picking up the phone themselves or complaining to the producer about how they had been portrayed. The kids calling in were usually just disembodied voices on the producer's speakerphone, but a caller did appear in one episode and was shown to be animated as well. This segment is usually cut in syndication.
On an interesting side-note, these segments usually consisted of the producer apologizing for the creative liberties taken with reality (such as explaining that, yes, the heat given off by the lava should've fried the kids by itself), but pointing out that, had they been completely realistic, there would be no story. It was, from an educational stand-point, a refreshing break from most cartoons.
The only true Animated Actor in these segments is Liz, Ms. Frizzle's class lizard, as opposed to the producer character, which is only seen in these segments.
Phoebe also did the Q&A session once for the desert episode ("All Dried Up").
Utilized in an episode of Eek! The Cat, wherein Eek discovers that the voice actor of his girlfriend is a large burly man, and that he himself is voiced by an old lady.
House of Mouse, it's basic premise is "The cast of every character in the Disney Animated Canon goes to a night club run by Mickey and Co. to watch shorts staring... Mickey and Co." In one episode, Mickey thought his position as host was being threatened, which sent him into a panic because "Showbusiness was the only thing he could do."
In another, Donald Duck is forced to stall for time on stage (including changing the name and trying to impress the audience) while Mickey, Goofy, and Minnie go film a cartoon, on account of the one they intended to show getting lost (or stolen-an occasionally recurring element was the "Show Must Go On" clause in the lease, which says that landlord Pete, who wants to bulldoze the place to put up some other venture, can't do a thing as long as there's an audience and a show, so he tries varied and sundry plots to remove one or t'other, including stealing all the cartoons).
The guests are also hinted at being Animated Actors. In one episode, Cruella de Vil says "One movie and you're labeled for life", and no villain acts as evil in the House of Mouse as they do in their movies. They do silly things based on Flanderization, instead (i.e. Jafar is obsessed with lamps.)
Unsurprisingly perhaps, Family Guy does this regularly.
One episode had the musical number at the start of every show become completely messed up, with several of the cast suffering what should have been broken bones at the least, and one apparent death. Stewie jumps up in front of the camera and tells the camera guy to stop rolling, cut to the show.
Another episode had Stewie make an obvious Cutaway Gag set up and when nothing happened he asked if they had a clip to show. Yet another had Brian and Stewie discussing whether a person would be able to understand Stewie fully, which generally only applies to one-off characters and those less connected with the main cast, and a off-screen voice told them they were still filming, and then they got back on with the scene.
The episode "Business Guy" had Peter briefly looking at the camera and realizing they were filming before giving the episode's first line.
At least one episode of Family Guy ends with the cast stepping to the front of what is suddenly revealed to be a studio set of their living room, joined by the other characters, and doing a spiel to the audience.
One episode showed what happens during a cutaway:
Peter: Lois, I'm not goin' back to work tomorrow! That new boss has it in for me! He's meaner than a shifty salesman.
after Peter sets up this gag, Brian takes out a flask, Peter smokes a cigarette, Lois applies makeup, and Stewie, Chris, and Meg text on their cell phones.
Lois: You sure you got time to smoke?
Peter: Oh, yeah, it's an Al Harrington, it goes on for a while.
The Aardman stop-motion animated short Pib and Pog ends with the revelation that the two violent lead characters are merely actors playing roles.
Animaniacs is all about this trope, as is stated in the theme song ("We have pay-or-play contracts..."). The Warners and Slappy Squirrel will remind us of this in nearly every episode. The trope is invoked to the point that it's not always clear where "reality" ends and the show begins. (The premise would appear to be that the scenes at the Warner Studio lot are taking place in the "real world", while the "show" constitutes the skits; yet if Scratchnsniff, Ralph, etc. are also actors in the title sequence, that suggests even those scenes are scripted. What gives?)
One King of the Hill episode had Hank Hill end the episode by addressing the audience about the nudity of the preceding episode (a shot of Hank's bare ass). He claims as an actor he would only agree to a small amount of nudity if it was required for the story (but the FOX executives were pushing for a lot more). He then apologizes to anyone who was offended by his nude body.
Likewise, a clip specially animated for a blooper show featured a "blooper" wherein Luanne's top accidentally falls off during "filming" of a scene. Everyone chuckles and Hank teases that Luanne "already got the job."
There was a storyline that played during promos between the end of the second season and the beginning of the third involving a network plan to move the series to Los Angeles (Mayor Guiliani pitched for New York). After it was resolved that the show would stay in Arlen, the season-ending cliffhanger was discussed (Hank promised that the death of a main character would happen over his dead body).
This sort of thing was surprisingly somewhat common, where one of the main characters would show up during the credits to talk about a certain subject, usually what the episode was centered around. For example, in "Keeping Up With the Jonses", a smoking related episode, Boomhauer talks about the dangers of smoking. "The Perils of Polling" likewise had Hank talking about the importance of voting.
A late-entry Terrytoon, "It's a Living", starred aggressively cute stock character Dinky Duck, who's had enough of his cartoon routine and walks off the movie screen, 'Purple Rose of Cairo' style, for the greener pastures of television.
The Total Drama series, although they're more Animated Reality Show contestants (other shows in the universe, including Celebrity Manhunt, suggest that it is an animated world where the contestants are in a regular reality show).
Dave the Barbarian would occasionally conclude a show with several of the characters sitting off-set in a typical film studio to address the audience about some issue they may have had with the episode. Of course there was never any serious talk given.
The Cleveland Show shoots "Cleveland Live!", the first episode of the second season as if it were a real-life sitcom. The episode begins breaking down halfway through when Roberta gets drunk on the set because her role got cut from the episode, Cleveland falls off the set and hurts himself during a skit, and at the end there are two Rallos, implying that like the Olsens on Full House they were Making Use of the Twin.
Occasionally done on Jimmy Two-Shoes. One short had Jimmy and Lucius host a blooper reel.
Daria has done this on occasion; for example, the TV movie "Is It Fall Yet?" features fake outtakes at the end, including scenes like a character reading a script for the movie. The "Sarcastathon" marathon that aired before season five also includes introductory segments (included on the DVDs) where Daria and Jane know they're in a show and even quip about the segments' limited animation.
Done in many episodes of Garfield and Friends. Once in "Flat Tired" Garfield is sleeping and the director tells him to wake up and start the cartoon. He says he's tired and the director picks Odie to replace him in the episode. Garfield watches the cartoon on tv as it happens and makes comments. When Odie gets in trouble the director asks Garfield if he's going to do anything. Garfield agrees to help, but insists he get guest star money
Happened a lot in The Twisted Tales of Felix the Cat. For instance, in one episode Felix washes up on a beach and has an argument with a clam about whether or not he should get up and explore the island. The clam pulls a script out of its shell and declares that Felix has to explore the island, since it's in the script. Felix sighs and gets up to instigate the rest of the episode.
The basic premise of Drawn Together - every character is an expy of a famous animated character, and they're all cooped up together in a "reality" show.
Crops up occassionally in Taz-Mania. The most prolonged example occurs in "But Is It Taz?", where Taz gets fed up, rips up his contract and storms off the set. The Platypus Brothers spend some time auditioning replacements while Taz gets a job at the local Burger Fool.
This occurs at the beginning of the U.S. Acres episode "Kiddie Korner", after Aloysius learns they are doing Dr. Schivago, and says they can't do it because U.S. Acres isn't Masterpiece Theatre.
Many of the characters in Bonkers are actors who work in cartoons. The title character, a Toon bobcat, is a former actor turned law enforcement officer when his show was cancelled.
When Gene Deitch took the helm at the stodgy Terrytoons studio, one of his first cartoons had regular Dinky Duck quit in mid-cartoon, walk off the screen and out of the theater for the greener pastures of television work. By story's end he's had enough of it and returns to where he started.
In thisKim Possible promo, Drakken flubs his lines, Shego mocks him pretty much the same way she does "in-character", and Kim tries to get everybody back on track.
We discover in one episode of Beetlejuice that the cartoon is actually a (reality??) show on the Neitherworld Network, where Mr. Monitor works. After Mr. Monitor cancels BJ's show, BJ goes to work in the mailroom. He quickly takes the opportunity to steal some show ideas from a colleague and is rapidly promoted to Mr. Monitor's supervisor. He eventually gets demoted after running out of ideas, and ultimately gets his old "show" back.
Subsequent episodes sometimes came back to this idea and featured BJ hosting shows on the Neitherworld Network like MonsterPiece Theatre.
Two episodes of Darkwing Duck are based on this. One episode has Darkwing driving to the studio to prevent a Corrupt Corporate Executive from retooling the show (using Green ThumbAnti-Villain Bushroot as the new hero) and another episode deals with Darkwing writing up a comic book of his exploits and trying to sell it to a studio (it's rejected, but then Darkwing decides to send it to Disney) Everyone in the first episode is clearly an actor, but the second episode still has everyone in character.
This was technically always the case in the Barbie movies, but was only made official with Barbie: A Fashion Fairytale.note Which officially "retcons" the previous movies as having always existed in the Barbie universe, something that had always been implied. Until then, it had just part of the toyline's theme of Barbie having many careers, acting being one of them.