Whenever Bugs made a cameo appearance in another WB cartoon, the Merrie Melodies theme would cue up. He made an appearance in the George Pal Paramount Puppetoon "Jasper Goes Hunting," and the accompanying music was an ersatz variation of the Merrie Melodies theme.
Stalling was addicted to the used of Leitmotif, to the point that Warners animators joked, "Now, don't draw anything with a red pencil, or Stalling will give us 'The Lady in Red'!"
American Dad! has Stan's high school bully Stelio Kontos, who not only has a distinctive theme song played over every scene he appears in, he also carries a stereo around with him so that he can play his own theme song in place of actually speaking.
In Batman: The Animated Series, nearly every character, and every villain, had a leitmotif that was associated with them. Nearly every leitmotif, with the exception of the Joker's, originated in the title card for their premiere episode, and thus was featured strongly in the soundtracks of the episodes they appear in.
The episode "The Strange Secret of Bruce Wayne" gives the best example when The Joker, Two-Face, and The Penguin all exit a plane one after the other. Their respective themes play them off the plane, and blend seamlessly from one to the next.
Batman's theme himself is quite possibly one of the most memorable ever conceived for the character, arguably moreso than Danny Elfman's from the 1989 movie (which Shirley Walker's interpretation has many themes in common with).
Two-Face, for example, had a short but haunting two/three tone melody motif that would play every time he was onscreen (whereas most had their motifs simply played in larger scenes), even if the episode had nothing to do with him (IE, if a character was passing by his cell in Arkham).
The Joker, whose theme never played in a title card, had a whimsical but subtly creepy theme (plays at the end) that had a different version of it played (ofttimes with completely different instruments) for every episode he appeared in.
THIS video even has a subtle almost fourth-wall breaking nod to the theme of the animated Joker at the end.
This idea was used expertly in "Perchance to Dream". As Wayne races to the top of a church belltower the Mad Hatter's theme is being played by heavy brass (as opposed to light, cheerful flutes as per norm) despite the fact he's doesn't appear to be involved in Bruce Wayne's current plight. Discerning viewers get to notice this, figure out the twist, and feel happy.
In the origin story for Harley Quinn, the Joker is whistling his theme the first time they meet.
Batman Beyond mostly had its own music, but would occasionally do a callback to leitmotifs for the previous series, and in the episode "Out of the Past," when Bruce Wayne is temporarily restored to youth and vigor, he and Terry take out the trash to a rock-anthem treatment of the BTAS main theme.
The very first time this callback happened in Batman Beyond was in the first episode during the last time Bruce appeared wearing the batsuit Terry would wear years later.
Another notable - and very effective - callback to the original BtAS score occurs in the episode "Disappearing Inque," when the aged Bruce Wayne appears in a suit of Batman Powered Armor to come to Terry's rescue.
Yet another one; in the episode "The Call", a brainwashed Superman ends up fighting Terry, and then follows him when he gets on the Batplane and goes to Mach 3. As this goes on, his theme plays in the background, but there is something wrong about it, somehow...
It gets slowed and "darkened" to show that Supes is Not Himself.
In Superman: The Animated Series, the first line of the theme music serves as Superman's Leitmotif. Used to creepy effect in "Identity Crisis" with The Reveal of Bizarro, whose leitmotif has the first three notes in reverse order, and adds instruments to "darken" it.
In the episode "A Little Piece of Home", a thug named Joey had an awesome jazzy theme. It's a shame he only appeared in one episode.
In SpongeBob SquarePants, various locations have leitmotifs, most notably Mrs. Puff's boating school, the Barg'n'Mart, and the Krusty Krab. A smaller number of characters, most notably Mermaidman and Barnacle Boy, have their own themes as well.
This theme tends to get associated specifically with Squidward.
Huh. I'd have thought "Arab Dance" from "Nutcracker" would be more fitting.
Plankton had atleastthree in his season one appearances, all of which were performed by surf punk band The Surfdusters. He also has an in-universe example to accompany his Evil Laugh: He actually pulls out a record player to provide villainous music as he laughs maniacally.
The song called "The Lineman" existed before Spongebob, but kids have it embedded in their heads as the "Mermaid Man and Barnacle Boy theme."
In Justice League and Justice League Unlimited, Batman, Superman, Wonder Woman, the Flash, Green Lantern and Green Arrow all have their own musical motifs (Batman and Superman retain their melody motifs from their respective solo shows). Green Arrow even hums his own theme while swinging forth into battle at one point (and it was AWESOME and hilarious at the same time). Here is a compilation.
In Teen Titans, Mad Mod gets a noticeable musical motif. Terra as well.
In Inspector Gadget, every main character had his or her own theme. Gadget's was the main theme of the show, which played in almost every scene as a recurring part of the soundtrack. He also whistled it sometimes. Penny also had a fairly thorough theme playing in each of her scenes. Brain, Quimby and Dr. Claw each had a short motif that played when their scenes began, while Claw had a longer theme that played in the background afterwards.
Kim Possible has a few pieces of music used in specific situations, including Rufus's theme, which was put to lyrics as the "Naked Mole Rap" in a third season episode. All the major characters also had their own individual Leit Motifs which would occasionally pop up. The most frequently was Kim's theme, a high-energy guitar variation on the show's theme tune. Ron had his own (somewhat sillier) up-tempo guitar theme, although it shows up less often. Dr. Drakken is accompanied by looming strings, although it's often replaced with a flute when he's being silly. Shego receives a techno beat. Killigan is aided by the bagpipes, while Monkeyfist often appears accompanied with a monk-like male bass chant.
X-Men: Evolution has these for several characters, from full-on Anime-like image songs (Toad) to sinister little villain tunes (Magneto). Magneto's piece is even based on the music playing at Auschwitz at the beginning of the first live-action film. Angel, the only character in the show to operate as a traditional superhero, gets very heroic music.
Scott and Rogue (as a duo) get one too. It's weird, seeing as they aren't the Official Couple, but nonetheless beautiful music.
The Boondocks has Uncle Ruckus whose presence is always preceded by a trombone tune, reminiscent of Jabba the Hutt's theme. Although not heard until Season 2, it is quickly noticeable.
Every engine in Thomas the Tank Engine had their own theme in the early years of the show. Several events were also accompanied by a recurring theme (eg. when an engine crashes or becomes a Runaway Train).
And when she's in the middle of her Villainous Breakdown, we're treated to a disjointed amount of strings.
Any time we see the Blue Spirit, we hear a "Tsungi Horn", a fictional instrument in the show made by altering the sound of a duduk. When he is engaging in stealthy moves, this changes to a vaguely bell-like sound, crescendoing and decrescendoing in time to the understated action. (This theme was even played briefly when the still-unpurchased/unstolen Blue Spirit mask went past Zuko and Iroh on a cart, before the character of the Blue Spirit emerged. You ALWAYS know when the Blue Spirit is about!)
There's a cool moment in "The Boiling Rock" where Zuko and Sokka's leitmotifs are played together, signifying that they have become BFFS.
Zuko and Azula share a Leitmotif because they're brother and sister. They just arrange it differently and use slightly different instruments so his sounds determined and serious, and hers sounds girlish, yet creepy.
Sokka and Yue also have one, though it may or may not count as a leitmotif since it is essencially an instrumental version of the song Iroh sang in the episode Yue was introduced.
Ozai's leitmotif once he's introduced as a character in the third season sounds scary and menacing, but in a cold, calculating way. When he announces his plan to burn down the Earth Kingdom, some insane-sounding violins are added to it.
ThunderCats uses leitmotifs extensively. Each major heroic character has one (Panthro sharing his with the Thundertank), as well as having leitmotifs for danger, warnings, certain locations, and battle.
The characters get new leitmotifs in the ThunderCats (2011) reboot as well, the ones most often heard being Lion-O's the Thundertank's and Mumm-Ra's.
Phineas and Ferb has leitmotifs for various characters, including Candace (a variation on the "universal taunt" melody), Perry (a James Bond-inspired jazz theme), and Dr. Doofenshmirtz ("Doofenshmirtz Evil, Inc.!").
Doobie doo ba doo wah doobie doo ba doo wah PEEEEERRRRRRRRY!
These are used more often than you might think. For locations and catch phrases, for example. There's the "I know what we're gonna do today" pling, "What cha'doin?" cute music, spy music when Perry gets his mission, Bavarian-like music whenever Doofenshmirtz appears, and overly sad music when Doof has a flashback. There are lots more.
This actually happened by accident; the composer used the Cape Fear music to drive home the parody and add to the cheesiness of the suspense. It wasn't meant to represent Bob, but since he was the bad guy, it naturally played only when he was onscreen. The producers ran with it and now it's quite explicitly his theme, always playing when he first appears in an episode (just in time for Bart and Lisa to shout in unison: "AAAAAAH! SIDESHOW BOB!")
With (usually) each appearance, alien antagonists Kang and Kodos can easily be recognized by a haunting, other-worldly theme that follows.
Whenever villain Charles Montgomery Burns is in his mansion or is planning another evil scheme, you can bet you will hear that sinister music reminiscent of Citizen Kane's opening sequence. Release the sounds!
Two words (and three notes): Fat Tony.
When Futurama featured early appearances of the Omicronians (invading Earth), a brief leitmotif can be heard.
South Park has been known to do this with some one-shot characters, one notable example being Blanket from the episode "The Jeffersons".
Adventure Time does this sparingly, mostly for one-shot characters. For example, Lemongrab's leitmotif is a quiet, out of tune harpsichord and/or piano played at an irregular rhythm, and it often gets louder when he yells. His leitmotif is pretty subtle, but it helps to underscore just how bizarre and awkward his character is. It almost sounds like there are mistakes in the music, or that the piece doesn't make sense- which is fitting, because he is a failed science experiment.
The 1970s British pre-school cartoon Bod had a prominent theme tune for each of its characters, all of which were short, simple and catchy so that kids would memorize them quickly.
Several characters and situations in Ben 10 have these.
The hoverboard music.
In The Venture Bros., an absurdly dramatic theme song always accompanies Dr. Byron Orpheus's theatrical outbursts, which comprise a majority of his dialogue. The Monarch has similar dramatic music that plays whenever he's being particularly commanding or villainous: "MINIONS!"
Hank and Dean have a quick musical sting (the last five notes of the show's theme song)that plays whenever they get excited about something, or when they do the "Go Team Venture!" salute.
Tiny Attorney has a laconic, slightly off-key banjo-picking motif that plays whenever he goes into his 'simple country lawyer' routine in a thick Cajun patois - which draws no attention from his being a baby-size adult conjoined to the torso of an inbred simpleton.
In Doug, several characters, including Doug's sister Judy, bully Roger Klotz and twins Al and Moo have their own leitmotif.
There were several situation/emotion leitmotifs, e.g. love, sadness, anxiety, etc.
Freakazoid!'s leitmotif plays virtually whenever the camera cuts to him.
Every distinct character or situation gets its own leitmotif. Notably, The Huntsman.
In Animaniacs, whenever the characters of one segment made a cameo appearance in another they would be accompanied by a few bars of their own segment's theme (the Animaniacs theme served as the motif for the Warner siblings, specially when they appeared in Pinky and the Brain and Freakazoid!).
Slappy Squirrel's leitmotif was "Humoresque" by Dvorak.
Certain actions warranted their own theme music too. For example, a character eating was usually accompanied by "Shortnin' Bread" and a character cleaning something would be accompanied by "Here We Go 'Round the Mulberry Bush". This is an obvious callback to Carl Stalling's work.
The Twins in Superjail! have a eurodance-esque beat whenever they appear or are about to appear on-screen.
Several characters of Static Shock have their own leitmotif.
Digeri Dingo has his own background music that usually when he's plotting something or saying a funny line in Taz-Mania.
Every time Cho-Cho from Histeria! spoke, she'd be accompanied by the "Chopsticks" music.
Tai Lung from Kung Fu Panda gets his own theme, natch. What is interesting is that while it plays the first time we see him (complete with Vachir's solemnly intoned "Behold Tai Lung"), and at several points throughout his incredible escape sequence, with all the gravitas, power, and chilling effectiveness you'd expect, it also appears in a subdued, pensive, even sorrowful tone when he stands before his former master just before their climactic battle, and again in a darkly tragic and heartbreaking iteration (complete with choral rendering!) as Tai Lung is attacking his father with his fists on fire. (As if we needed any more reminders of his Freudian Excuse and "Well Done, Son" Guy status—this is during his "All I ever did, I did to make you proud!" speech.) And a heartwarming, sweetly playful version is heard during the flashback to his Start of Darkness, when we see him as an adorable cub with Shifu and learning kung fu.
There is also a theme just for Oogway, as well as another that seems to be for kung fu. And one just for Po. If one listens to the music during Po's fight with Tai Lung, each of the two themes will flourish as its character shines. And as Po starts to kick Tai Lung's butt legitimately, the "Hero" theme from the dream sequence at the beginning of the film comes back.
As befits her modus operandi, Ember McLain of Danny Phantom was the only villain with her own theme, which subsequently became her leitmotif; savvy viewers later on could tell that she was back from the fact that the new radio broadcasts were playing a New Age version of it.
The slow ragtime music for Muriel, the jumpy fiddle that usually goes with Eustice, the weird beat that plays whenever Courage springs into action and...Freaky Fred's theme, which has even gotten some remixes.
The oriental sounding background music that usually follows Di Lun, ya fool.
Megas XLR uses its musical themes to great effect. The utterly metal "Blood Shot" plays during some heavy brawls, the ominous-sounding "Intrepid" plays whenever the cast is in danger and "Short Hair", Coop's battle theme usually play when he's kicking alot of ass! Several characters, both recurring and non-recurring, have themes of their own.
The Glorft, whose appearance is usually accompanied by their menacing militaristic theme "Collision Course".
Lampshaded by Family Guy. Peter Griffin is awarded wishes by a genie. His first wish is for his own theme song. It doesn't work until he starts marching around the house.
Also parodied when Stewie follows Peter around playing a tuba whenever he walks anywhere.
Duckman had a recurring theme for villain King Chicken.
Fluffy and Uranus had a cutesy glockenspiel/celesta tune play whenever they usually appeared, too.
Garfield TV specials and shows would mark the entrance of Odie with a brief motif based on a hunting charge.
Wade the Duck's entrance would often be marked by a few bars of "Wade, You're Afraid".
And Roy the Rooster's appearances were underscored with Schumann's 'The Merry Peasant'
Binky the Clown had his own theme which sounds like an instrumental version of his name.
Wilbur Cobb's appearances in 'The Ren & Stimpy Show''' were underscored with passages from Prokofiev's 'Peter and the Wolf' - specifically, the 'grandfather' and 'cat' themes.
The Pink Panther had his own theme in all (but one) of his cartoons.
The Inspector, Crazy Legs Crane and The Ant and the Aardvark also had their own themes appear often in their shorts.
In The Mask cartoon, Stanley's dog Milo would mark his appearance with a little 5-note tune.
Parodied in an episode of King of the Hill, where Dale buys a keyboard and makes up leitmotifs for the guys. During a later scene, we hear Dale playing Boomhauer's theme, which Hank uses as an excuse to leave an awkward conversation with Peggy.
Some, if not all, of the main characters from My Little Pony: Friendship Is Magic have their own. Applejack's is a bluegrass country theme, Rarity's is a classical-sounding piece featuring a harpsichord, and Rainbow Dash's is a rock riff. Discord has a distinct tune that usually plays whenever he warps reality.
The characters have also been noted to be seen idly humming the main theme.
Some characters in Recess have one, such as Butch, who usually has a creepy tune playing when he's around. The kindergarteners always have tribal music playing when they're around, and then there's the music that plays whenever T.J. has a plan.
The Ashleys, the Diggers, and Hustler Kid all have their own themes as well. King Bob has one too, but it doesn't show up very often.
Spinelli has one as well, but it doesn't show up often.
Invader ZIM has "Tak's Theme", which includes a special section for Tak's robot sidekick, Mi Mi. Tak was only in one episode, but since she was planned as a recurring villain, who knows how long the theme would have appeared?
There's also the little tune that plays whenever the Irken Armada shows up anywhere.
Mike Batt composed one for both the hero and villain side in The Dreamstone. The title theme "Better Than a Dream" and variations of are used for the Land of Dreams. While "The War Song Of The Urpneys" is used for Viltheed.
Poopdeck Pappy has one, which is an instrumental version of his song from "My Pop, My Pop".
Danger Mouse had two notable leitmotifs—a soft jazz rendition of the theme in opening scenes where he gets his assignment from Colonel K, and a silly lilting rendition when something, well, silly happens.
'Victor And Hugo' - Bunglers In Crime' also had variations on the theme tune within the episodes. Among severeal bars cropping up in otherwise unique passages, there was a slower stealthy version of the theme for when sneaky work was underway, a cool piano like version for calm moments and a slightly sadder version for when the brothers inevitably failed at something.
Avenger Penguins Almost any time Doom Tower is first shown in an episode a long eerie sting is heard. Also a sinister but slightly comical sting for whenever Mr. Doom laughs madly to close a scene. Any appearance of the Slush City ice cream parlour starts with an upbeat bouncy little sting.
As Told by Ginger had themes for some of the characters in the show. Caral's theme and Blake's theme were some of the more memorable ones.
On CatDog, Winslow had a brief 4-note tune that usually played when he came out of his hole.
Lola had a Latin tune whenever she appeared.
While Motorcity usually has a rock/metal soundtrack, whenever Tennie shows up, there's this more beautiful, pieceful sounding piece of music. The Duke and Kane might have a leitmotif as well.
X.A.N.A. doesn't have one, but two themes in Code Lyoko and several variations of both.
Each of the Monks had their own in Season 1, rarely heard after that.
Also, in what could be Fridge Brilliance, Season 2 introduced us to the Group Anthem, an epic heroic-sounding theme for whenever all four of them stood unified. Perhaps symbolizing that they were more unified as a group after the character development they had through Season 1?
The villains were fond of these: Jack had a guitar riff, Wuya had an eerie mystical-sounding motif, Chase had a frightening gothic choir, while Hannibal Roy Bean... had a FIDDLE.
Then there's Dojo's flying themes: an Asian-sounding theme in Season 1, and a much cooler heroic-sounding one from Season 2 onward.
Steven Universe has theme instruments for pretty much every character, with Pearl being represented by piano, Amethyst being accompanied by a sort of beatboxing rhythm, and Lion has cat-toy sounds that go with his cute music box style theme. What's even cooler is that, when the characters fuse, their themes combine.