You know someone like this... It might be even you...
People who are planning to commit suicide often become this in the very final stage. This typically means they have made peace with the idea, have already planned out when and how to do it, and don't have much time left (It might even be hours). Anyone who has contemplated suicide or has depression who suddenly starts acting happy and calm may be in serious need of quick help.
Classic hebephrenia. Oliver Sacks described it in his classic The Man Who Mistook His Wife for a Hat with a doctor who was aware of her own condition. Her finding humor in everything was humorless, and a sign of a complete lack of empathy.
Clinical depression can invoke this: a sufferer can seem fine (even cheerful and friendly) to people who they come in contact with, but are really not.
Despite the popular perception of them being these, sometimes the mark of an all time great news-caster is the times they do let down their facade of journalistic impartiality, such as during John Glenn's Mercury Capsule launch when Walter Cronkite, getting a bit too caught up in the excitement to maintain the stoic facade, said of the rocket, "Go, baby!"
For a more recent example (both times) when Brian Williams was invited to play "Not My Job" on Wait Wait... Don't Tell Me!, he showed that behind the facade of somber, impartial journalism, he is actually sharp as a tack, and has a lightning-quick wit. When Williams is on The Daily Show, Stewart meets his match in Williams. For a rare example of him breaking the facade while behind the news desk was when he reported on the increasing popularity of knee replacement surgery, announcing that his (then) upcoming hiatus was because he was going to be getting such a surgery on his own knee.
Some news-casters, particularly those who also commentators, are more able to avert this trope, such as Keith Olbermann, and the late Paul Harvey, but it's a thin highwire to walk, as getting too impassioned in any direction with one's commentary can cross the line from showing oneself as human into letting loose with an unhinged rant a-la Network.
Speaking of Paul Harvey, he once reported on a person who was the only passenger on a Boeing 747 flying from Bombay India (as it was then still known) to Cape Town, South Africa. The flight-attendants waited on him hand-and-foot as if he was royalty. They lost his luggage. When the time came to reveal that little detail, Harvey cracked up laughing.
A similar case happened with Oprah Winfrey when she was a news reporter in her early days; she was fired from her newscasting job because she cried on camera when reporting a tragic story.
One of the reasons for the "grim Russian" stereotype is that in Russian culture, smiling is supposed to express actual happiness or amusement, not just be a polite gesture. Someone who who smiles at strangers is assumed to be either a Stepford Smiler, insincere, trying to flirt or finds their appearence funny. In the last case it might even be considered offensive. There is an old Russian joke about this: "Americans look at you with phony politeness. We look at you with sincere hatred". In fact, in Russian the term "American smile" is more or less synonymous with this trope.
Dolphins. They have that "smile" on their faces regardless of their mood. Unfortunately, many people think that because of their "smiles", dolphins are nice gentle creatures. They are not.
People of any gender who are or have been physically/sexually/emotionally abused in one way or another, often become this to hide their suffering from others.
Some people who are in the above situation put on this facade due to a feeling of isolation from other people. They can understand that abuse doesn't happen to most people, and yet they have been abused and are now one of those people. They understandably feel like they don't belong around other people, who they assume to be non-abused (therefore not like themselves) due to abuse commonly not being discussed. This also exacerbates the survivor's feelings of isolation. Hence, they'll put on an affable persona to cope with this, give themselves a basis on which to feel like they belong, and hope that this will become real.
Some abusers will outright force their victims to put on an act like this, as any slip up of sadness or anxiety could get a witness suspicious of something bad taking place in that person's life, leading straight to the abuser's exposure.
At most job interviews, the interviewee is expected to smile and look and act pleasant regardless of the dread and nervousness they're experiencing.
A good customer service representative or anyone else in the service industry who deals primarily with customers will, as part of their training, put on a Stepford Smile. Even if they're answering phones in a cubicle farm, they will be told to fake a smile so their voice sounds pleasant and friendly. Most superiors will demand that of these employees even if they're dealing with a terrible customer, with few exceptions (e.g. if there's a company mandate against dealing with people who are cursing at the employees). There is even a term for the condition that can result from this, the smile mask syndrome.
A "Stepford Laugh" is a common effect of marijuana. An uncontrollable laughter without any really pleasant distractions accompanying it.
When the marijuana laugh is real (everything is funny), it's sometimes called "Cosmic Joke".
Psychopaths are often Unstable, being superficially charming but having no real empathy and either do not respond or respond inappropriately and end up committing horrible crimes with the simple goal to receive some kind of emotional stimulus, which quickly becomes addictive. The term is Shallow Affect, and 'describes the psychopath's tendency for genuine emotion to be short-lived and egocentric, with an overall cold demeanor'.
Lot of people who try to live according to The Secret (and other similar...ideologies?) probably end up being Depressed, at least temporarily.
Only when taken to its logical extreme, which is not necessarily encouraged.
Barbara Ehrenreich talks in depth about these negative aspects of the positive-thinking movement in her book Bright Sided, specifically, directing attention to how cruel and callous it is to expect people who are at an all-time low point in their lives to act happy about it, going so far as to point out how it was a hallmark of propaganda in a lot of totalitarian dictatorships, including the Soviet Union, among others, and how cruel it is to tell people who are depressed that they have only themselves to blame for it.
Disney Theme Parks cast members typically will put on a Stepford Smile, no matter what kind of rude guest they may face. It's actually part of their job description to be smiling and welcoming towards any guest.
Six Flags team members do the exact same thing, which makes sense since they are Warner Bros. answer to the Disney Theme Parks.
The Westboro Baptist Church has a bunch of these people, most tragically in the children that have the misfortune to be part of it. During interviews they'll cheerfully talk about how you'll go to hell and reveal that they have no friends because everyone in school hates them due to their association with their church. The only time on TV that one person has slipped even a little, she was told by a reporter that if he thought that she was going to hell, he'd be sad. She cracked just a tad, then said she needed to go.
Traditional people on the island of Bali are required to be this. To show anything other than your "bright face" invites evil spirits and curses, and might make other people sick. Devastating tragedy strikes? Laugh as if it's the funniest thing in the world. Your friends know you are really suffering, but will respect you for staying "bright". If your facade breaks, they'll surround you and laugh at your "insanity". It's closest to Empty, as "what counts is to try not to feel." All your social interactions must be carefully stylized to avoid giving offense to someone who might put a spell on you. (You don't know what they're thinking, because they're Stepford smiling). The smoothly perfect serenity of the Balinese is a masquerade.
No matter how hard, troublesome, or disasterous a Filipino would ever experience, they smile and laugh in the midst of it. Of course, they smile for hopes and expectations.
Michael Jackson. Since childhood, he had to project an image of happiness even when he was personally very unhappy. The fact the song "Smile (when your heart is breaking)" was his favorite song of all time should tell you all you need to know.
" I think people felt safe enough back then that when they were fucked up, they said so. In the 90s there were a lot of mouthy, gobby women who were in disagreement with the mainstream, and it felt to me very much like we burst through a certain glass ceiling. Now everything has fallen back down a little, and itís almost like the Mad Men era, where women are supposed to be perfect. They can do their job, but they have to have children and they have to be good moms and they have to look pretty at all times and they have to be always smiling and it must be fucking exhausting."
In a nutshell, The Roaring Twenties could be depicted as this. Sure there are parties, liberated girls and illegal booze running around for relief, but as you can dig deeper, there's still that wartime trauma lingering on, an mask that hides closing banks, bubbling stocks, and scarce farmland, along with extremely bloody gang wars and incredible amounts of corruption if you're in Chicago. And also, take a good read about the fate of The Great Gatsby to see what it looks like in under nine chapters.
Sudden smiling or laughter is often a sign of distress for people with severe autism.
Several people will start giggling uncontrollably under extreme pressure or stress, partially as biology's way of trying to actually induce stress relief. Laughter is therapeutic, and can definitely help, but it's still not that great to suddenly crack up when you're about to give a big speech or when you're being yelled at. (Especially the latter - if the person yelling at you doesn't know you're prone to nervous laughter, they may take it as a sign of disrespect.)
Michelle Duggar, especially as the allegations of son Josh's molestation of his sisters unfolds.
Doug Walker's talked about being this, saying it took a while for him to even learn that it was okay to acknowledge sadness, because for so long he felt like he had to be happy about everything, and had to make other people feel happy about everything, and if he could do that then he'd be okay.