Stoner's wife, Edith, in "Stoner" by John Williams, goes through a phase of embodying this trope perfectly.
E. T. A. Hoffmann's Olimpia from The Sandman is very much a literary ancestress. Nathanael becomes besotted with her because she has a more "positive" attitude than his fiancee Clara, e. g. to his efforts at poetry. She is an automaton, constructed to be pleasant and thus unable to talk back or criticize.
The Other Mother from Coraline at first appears to be a very sweet, friendly counterpart to Coraline's real mother. As it turns out she is the Big Bad and it was all a facade to get Coraline to stay with her.
William Sleator's Others See Us has Annelise, who is well loved by everyone, including her cousin Jared, until he gains telepathy and realizes she's a Stepford Smiler of the worst sort. At one point he visits her mental landscape, it's an infinite sun-parched desert with her face as the huge sun, and the only other feature is a gigantic mirror, reflecting her face.
C. S. Lewis' short story "The Shoddy Lands" (PDF link here) also has its protagonist experience a telepathic vision of a Stepford Smiler's mental landscape.
Petunia Dursley is a fine example of the sort who initially seems to be her mask. Deathly Hallows suggests that her mask developed as the means to deal with her jealousy over her younger, "perfect" sister Lily getting magic and not herself.
Dolores Umbridge. Part of the reason this character is so effective is that she wraps her sadism and violence in an unnerving Stepford mask straight from her introduction in Order of the Phoenix.
The Goddess Media from American Gods by Neil Gaiman was like this in her true form. As the humanoid manifestation of The Media, when she wasn't possessing characters on television she was described as looking like the sickeningly sweet hostess one of those morning shows filmed in a fake living room.
Then there is Cersei Lannister, who tries to run said court. Becomes especially evident in A Feast for Crows, when she becomes a point-of-view character.
In a somewhat unusual example there's Asha Greyjoy. Asha has managed to become a leader in a culture which completely believes in Rated M for Manly and Testosterone Poisoning as being the right way to do things, while thinking that all women should Stay in the Kitchen. As a result of this she is secretly afraid that any slip up, mistake or failure will lead to her being forcibly removed from the position of authority she has fought to achieve. As a result she overcompensates and practices a tough girl catchphrase in an attempt to prevent anyone from even trying to do that.
Littlefinger seemingly has a particularly empty one, as Sansa, a pretty bad judge of character at the start of the series, sees right through it. It's not clear exactly what he is under it, but having the blood of thousands on his hands hasn't chipped his mask.
Countess Rostov from War and Peace is very much like this except when she's talking with her daughters. Pierre Bezukhov's wife Helene would be a subversion in that she goes from having no role in society except being beautiful (and smiling a lot) to one of the eminent hostesses on the Moscow and St. Petersburg scene after getting married.
Part of the reason Will Navidson moved his family into the titular house in House of Leaves was to get closer to his family, including his Stepford Smiler wife, Karen Green.
Lilith de Tempscire from Discworld is a Transferred Stepford Smiler. In Lilith's mind, life should be just like a storybook. As the witch in charge, politically, in the city of Genua, she likes things to be the way people expect them to be—i.e., cooks should be fat and jolly and bustle a great deal, innkeepers should have big red faces, toymakers should whistle and sing the whole day long and tell amusing stories to children, etc. And woe betide anyone who doesn't live up to Lilith's expectations; she makes certain that they suffer for it. To quote the book Witches Abroad, "Lilith held up a mirror to Life, and chopped off the bits of Life that didn't fit."
The first appearance of Ysabell, in The Light Fantastic, is when with a fixed manic smile on her face, she explains what it is to be Death's adopted daughter - stuck, never dying, in the house of Death forever, having been fifteen for nearly fifty years in a place where Time does not apply, with no mortal human beings to talk to - well, not for very long, anyway - and (with a long sharp blade in her hands) explaining how nice it would be if Rincewind stayed for a little while longer...note It is possible that this is another underlying reason why Death chooses to take ''Mort as an apprentice in a later novel - to restore Ysabell to the world in a halfway sane state.
Kaitlyn Werhner from the short story "Dark Red Mind". If you were to see her smile at you with those piercing blue eyes, run. Not that it would help you any.
Carrie, a classic case of Depressed, is actually a deconstruction in Finding Snowflakes as she smiles more to hide the pain from herself than others.
Annie Wilkes in Stephen King's Misery. She is a cunning, brutal and dangerously disturbed woman who hides her psychosis behind a cheery facade and kind smile, making her Unstable.
Erzebet Bizecka of Alisa Libby's Blood Confession is Unstable. She's a charming and beautiful young lady who's doing an excellent job at leading her country out of difficult times. Not to mention how kind she is to her servants. Of course, that's just a plot to get them into her dungeon where she kills them and bathes in their blood to preserve her youth and beauty.
Miss America in Haunted 2005 constantly behaves as though she is on camera, working so hard to maintain her flawless facade that naturally her breaking point is just as epic as those of the others.
It's possible to read Agnes Wickfield in David Copperfield as this, considering her life with an increasingly alcoholic, depressed father and an increasingly lecherous Uriah Heep, yet she never loses her smiling willingness to help others. As well, Miss Mowcher is classically Depressed (though she was originally written as The Grotesque, Dickens switched gears and made her a tragic figure who shields herself behind laughs, even at her own expense.)
Glinda from Wicked is essentially this. As such a high and mighty political figure she must keep a happy facade. Made even worse when her best friend, who happens to be the person she loves, is killed. She must act as if she hates her and tell pitiful lies about her. It's debatable if the musical or book version has it worse.
Felicity in The Idea Of Perfection by Kate Grenville is obsessed with appearance, to the point of avoiding frowning or smiling out of fear of developing wrinkles. More-or-less Empty, since there's nothing of substance underneath her immaculately groomed and beautiful appearance, and contrasting brilliantly with the much more flawed, but likable protagonists, who, along with the flaws, also have goals, interests, and drives, and who actually get things done in the end.
The original Pollyanna is Depressed. She maintains her sunny disposition in front of people but she is sad about losing her father and breaks down when she becomes crippled. She gets better though.
Jennifer North in Valley of the Dolls tends to be this. Harry Bellamy says of her "That smile is glued on." She's unfailingly warm and friendly to everyone and sincerely cares about others' problems, while hiding a life full of shame, lies, bitter disappointments and, finally, breast cancer. Most of the truth is not revealed until long after her suicide.
Mrs. Coulter in The Golden Compass. She puts on a friendly, trustworthy front, when in reality she's one of the leaders of an organization that kidnaps young children and surgically removes their souls.
In Death: Allika Straffo from Innocent In Death is either Depressed or Unstable. That's because she knows her daughter Rayleen Straffo killed her baby brother Trevor. She tries to act like everything's fine and okay, because she's afraid of Rayleen and what she might do.
The Irish short story "An Beann Og" (The Young Woman) follows a mother of two who politely smiles to anyone she sees and greets them kindly. She seems perfectly happy as she gets herself tidied up for her husband coming home for dinner but the last line of the story says she feels a small tremble of despair at the thought of her husband.
The titular character of Stargirl, of all people has a tendency to become Depressed when she's sad. She is normally an upbeat Blithe Spirit.
Angela Wexler has given up her dreams and ambitions to fulfill her social climbing mother's wishes, making her Depressed with a bit of Unstability, as she vents her frustration by setting bombs, including one aimed at herself.
Flora Baumbach is purely Depressed who smiles constantly to hide the pain from her husband leaving her and her daughter dying.
Elin from Of Fear and Faith is a genuinely happy person for the most part, but she hides severe and traumatizing personal scars behind her cheerful smile and refuses to show anyone else that she's in pain.
Kiina in the Novelization of BIONICLE: The Legend Reborn. In the animated movie, she's a Genki Girl, even though the prequel book depicted her as a mean and argumentative person who got into bar brawls, barely had any friends and would have loved nothing more than to leave the planet. In TLR, when the appearance of Mata Nui presents an opportunity to do so, she adopts a friendly and cheerful persona to get close to him, fearing that if Mata Nui knew what life on her planet was like, he'd give up all hope and never try to help them.
Shallan Davar of The Stormlight Archive is revealed in Words of Radiance as a combination of Depressed and Unstable. Due to her Dark and Troubled Past, she must use Lightweaving to repress her true feelings in order to function. Arguably a Deconstruction, as Shallan's feigned chirpiness is not portrayed entirely as a bad thing; in one defining moment, Kaladin is humbled and awed by the fact that even though she has been through as much crap as he has, she can still bring herself to smile instead of becoming a Perpetual Frowner like him.
Amy Elliot-Dunne in Gone Girl is Depressed, having always resented the pressure her parents put on her to be the "Amazing Amy" of their books. Her increasingly unhappy marriage to Nick later on only made the pressure worse. It eventually turned her into The Sociopath and a textbook example of the Unstable.
Fellow Man by Norwegian author Olav Duun has Tale, the wife of sociopathic and Manipulative Bastard Didrik. She copes with her nasty husband, and keeps appearances up by smiling. She borders on Stepford Snarker as well. She lampshades it early on:
I am glad I have my grin. In this house, you either grin, or weep.
The Hunger Games: Katniss eventually realizes that Effie Trinket's shallowness is a defense mechanism which is her way of coping with her role in the Games.
In The Secrets of Drearcliff Grange School, "Perky" Palgraive smiles all the time, no matter what the provocation (and several of the prefects consider it a challenge to wipe the smile off her face). This is because she's a Meat Puppet for a Puppeteer Parasite that hasn't entirely got the hang of fine motor control or the nicer points of human interaction.
Early on in Redeeming Love, Angel acts like her cushy life as the city’s highest-end prostitute is all she could ever want, flippantly laughing off the hero’s suggestions that she might want to escape it—when in fact she has no control over her own life, is deeply lonely, and loathes every minute of her work. As the novel progresses and she is taken out of her “comfort zone", it becomes obvious that she’s really a deeply bitter and cynical Broken Bird.
The title character in Edward Arlington Robinson's poem "Richard Cory" is a male Stepford Smiler, a rich, elegant, successful man who is envied by everyone around him. His secret unhappiness isn't revealed until the last lines: "And Richard Cory, one calm summer's night / Went home and put a bullet through his head."
Aaron Armstrong being this is the whole point of "Three Tools of Death" by G.K.Chesterton. Discussed In-Universe by Father Brown: "Why couldn’t they let him weep a little, like his fathers before him? His plans stiffened, his views grew cold; behind that merry mask was the empty mind of the atheist." He committed suicide.
Many of the characters in Tangerine fit this trope. The three most glaring examples are Joey, Erik, and Paul. Erik is probably the king of this trope.
The genuine people in this book are found by exiting suburbia (achieved by the middle school getting sucked into hell a sinkhole and the kids being spread out) and rubbing elbows with tangerine farmers like Luis Cruz, and other "real" people.
David Foster Wallace's short story "Girl with Curious Hair" is narrated by a mixture of Empty and Unstable. One of the narrator's "punkrocker friends" tries to emotionally engage with him while high on acid and is deeply disturbed upon realizing that there is nothing behind the mask.
Xenophilius Lovegood tries to conceal Depression in Harry Potter and the Deathly Hallows when Harry, Ron, and Hermione come to his house. He pretends that everything is well and good and that Luna herself is only out getting ingredients for soup when he's actually been afraid because the Death Eaters have Luna and have threatened to kill her unless he captures Harry Potter for them, which is just the thing he's attempting to do.
Albus Dumbledore, best shown in Harry Potter and the Philosopher's Stone when he lies about seeing himself holding a pair of socks in the Mirror of Erised (an artifact which reveals a person's deepest desire).
Frank Chalmers in Red Mars is somewhere between Depressed and Empty. Coming from a poor family, he's almost entirely consumed by his ambition to succeed. At one point he looks at his life and realizes that all of it is a facade created to impress his superiors, and that he doesn't really have a self.
Telemachon from Black Legion is mostly the Depressed kind. To the outsiders, including emotion-reading Khayon, he's your typical Emperor's Child — indulgent, full of contempt and with no standards. Inside, however, he's Desperately Looking for a Purpose in Life and has a soul of a poet. When Abaddon proposes him to join the Legion, he promptly jumps ship.
Discworld serial killer Carcer is deceptively cheerful and innocent-looking, with his smile putting people off guard...until they look in his eyes and see the monster behind the mask. But of course then you've taken your eyes off his hands, and at least one of them is holding a knife by now. He's the kind of guy who would murder a man for a loaf of bread and then stand over the corpse saying 'Who, me, guvnor?' and almost convincing you.
Todd Bowden from Stephen King's "Apt Pupil" maintains the image of a cheery all-American golden boy even while he's blackmailing the neighborhood Nazi-in-hiding into telling gruesome concentration camp stories. It's all downhill from there.
Harold Lauder from The Stand becomes Unstable after he finds Fran's diary and goes crazy. He then starts smiling to hide the fact that he hates pretty much everyone, including himself. Another character later notes that when Harold is not smiling, he looks insane.
Mr. Happy was like that at, at least the start of Mr. Men A Christmas Carol to help and keep Mr. Mean happy. Ironically, this book was made shortly before The Mr. Men Show came on air...
In Luke Skywalker and the Shadows of Mindor, Luke is subjected to a terrible vision of what it's like to live through the heat death of the universe; when he comes out of it he's... different. He becomes nihilistic and depressed, believing that love and friendship are just tools people use to manipulate each other, and that it's pointless to save anyone. However, he's still Luke Skywalker, and he makes the conscious decision to act exactly like he did back when he still thought life had value and meaning, in the hopes of Becoming the Mask, longing to believe the happy lies again. Eventually he does get out of that mindset.
In Evgeny Zamyatin's dystopian tale We, the totalitarian government works towards making its subjects as machine-like as possible: perfectly scheduled and mapped lives, synchronized movements of multiple people, and lack of names in favor of numbers. This agenda is ultimately crowned by "The Great Operation" in which the human brain is irradiated by rays, that completely and irreparably strip a person of his imagination. One of the effects (aside from turning a human being into an obedient and ever-happy shell of a man) is a perpetual grin on the subject's face, as he now thinks "smiling is a natural expression of human face". In the end, D-503, the protagonist, is subjected to the Operation.
Theon Greyjoy. Everyone comments on how he is always smiling or snarking, as if he hadn't a care in the world or knows something nobody else does. And, almost everyone is to some extent creeped out by it because he keeps it up even when it's grossly inappropriate and often downright morbid. Sure enough, he's got issues even he didn't knew he had, as a result of being kept as a hostage half his life in a situation where everyone maintained a polite fiction of him being a "guest" and ward — i.e., someone whose life was constantly being implicitly threatened, who was supposed to pretend he didn't know that. All things considered it's probably not too surprising he turned out to be a complete basketcase.
Varys and Littlefinger (the resident Magnificent Bastards) are this, and in both cases, it's implied to be more than just a mask to wear in public...
The Summoning has Simon Bae, who for his introduction in the first book seems like a totally normal, cheerful kid who just so happens to be locked up with his older brother in a group home for mentally unstable kids. No matter how gloomy and angry his brother Derek is, Simon always remains upbeat and positive, until Derek snaps at him for doing nothing to help find their missing father. This is the first thing that breaks Simon’s happy mask, and he admits that having to pretend to be content all the time when he really wants nothing more than to run away and find his dad is killing him inside.
And again in The Reckoning, when he takes Chloe on a date, only to discover that his suspicions about her actually having feelings for Derek and not for him are true. He admits to having ignored it so he could just keep trying, but upon finding it out for real, his smile once again breaks and he actually leaves Chloe behind in the forest to go be alone for a while.
Could Derek count as well? He wasn’t exactly smiling, per se, but it’s implied that he tried to hide his own feelings about Simon and Chloe’s going on a date and act like nothing was wrong...
The Belgariad/The Malloreon: Silk, AKA Prince Kheldar, is asked why he's always laughing at life. His answer: "I've taken a good, long look at the world, and I concluded that if I didn't laugh, I'd probably have to cry."
"The Love Song of J. Alfred Prufrock" by T. S. Eliot: J. Alfred Prufrock attends parties and formal events to try to be accepted by his peer group but ultimately feels dead inside, and that he has never done anything significant with his life.
Bryce from Flipped comes off as Empty to Juli's family, but is really Depressed and hides how repressed he is exceedingly well.
A Spanish-language poetry, Reír llorando (To laugh crying, in case you needed more hints), is about a man who has everything he could want, but still feels empty and depressed, visit a doctor, who tells him to go see a famous comic named Garrick to cheer up. Then the man reveals he is Garrick and asks for another remedy.
Guy Montag from Fahrenheit 451 has a forced smile gripping his face at all times, even when he goes to sleep. It takes him some time to realize that his "happiness" is not real and actually masks deep unhappiness.
The Lord Ruler thinks he has claimed laughter and joy for himself. I'm disinclined to let him.
The narrator of Langston Hughes' poem "Minstrel Man" is Depressed.
Mixed Gender Examples
In Soon I Will Be Invincible, the Champions are a cracking facade of glossy superheroism concealing bulimia, pain killer addiction, and the usual shenanigans. Twisted later when the apparent Bulimia and pain killer addictions turn out to be food allergies and life-sustaining medications in keeping with the "mundane lives of superheroes" theme.
Arguably the entire tree-dwelling Kindar culture in Zilpha Keatley Snyder's Green-Sky Trilogy. The descendants of a group who decided that humanity's past misdeeds were best forgotten, they consider negative emotions (lumped under the heading of "unjoyfulness") inappropriate and best kept suppressed.
Vincent and Carrie Raymond (in Geoph Essex's Lovely Assistant) are astonishingly warm and cheerful Beautiful People, though their plans to summon a galaxy-sized monster and destroy the world places them squarely as Unstable. Jenny even thinks about the Stepford effect by name.
The main cast of Jodi Picoult's Nineteen Minutes (Barring Peter, Jordan and possibly Patrick)
Josie is Depressed, pretending to be the perfect Golden girl whilst simultaneously having to put up with an abusive boyfriend, neglectful mother and niggling suspicion that if she stops smiling for even a second, everyone will realise she's nothing special.
Matt is Unstable, pretending at first to be the perfect boyfriend, until he turns out to be an abusive jerk.
Alex has to act perfect 24/7 for the sake of her job whilst the strain tears her apart.
Lacy has to deal with the fact that one of her children turned out to be a druggie whilst the other (Peter) went on a killing spree.
Lewis (Lacy's husband) has to deal with the above whilst being a happiness economist (Meaning it's his job to work out the mathematical value of happiness)
Selena gets off relatively easy, only having to deal with racist idiots.
Dee and James from The Hearts We Sold both qualify. Dee is the more obvious example, putting on a happy face whenever she visits home to avoid inciting further abuse from her father, but we later find out that a lot of James' silliness and adventurousness stems from the fact that he has a brain tumor.