And why it's not quite as easy as it sounds.
We've all met them. Usually male, although the Evil Empress or Queen occasionally gets a look-in, the Evil Overlord, whether the CEO of Evil Inc. or the Emperor of the Galaxy, or a petty prince of some forgotten nation in Fantasyland, is something of a staple in science fiction and fantasy. Usually he, she or it exists mostly to be overthrown.
When you come down to it, it's usually pretty easy. Not necessarily easy at the "toss a trinket into a volcano" level (yes, I know I'm oversimplifying. Shut up.), but there's a big Final Battle of some description, the Evil Overlord dies, and all is happiness, sweetness and light. As often as not, the Evil Overlord is some kind of kludged-up metaphor for the hero's journey to some kind of enlightenment (something the hordes of Tolkien-imitators usually fail to notice is that Frodo did not gain 'enlightenment' per se. He was irreparably wounded by the trials of his journey, and ultimately unable to remain in/on Middle Earth. There was a happy ending, but it wasn't for him.).
So why do tyrannies in the real world last so long?
Leaving aside the obvious "they're not really that bad" answer that I'm sure people are thinking (if you are, you're wrong. Read on MacDuff), the most atrocious regimes of all political flavors have lasted a long time for reasons that can be illustrated by two things.
First a recent news report about the execution of a North Korean man for - officially - revealing state secrets. The state secrets in question? Work conditions at his factory, and the price of food. Who he revealed them to? A South Korean friend. Let's just consider that for a moment. How often do you bitch about your job, or about how much things cost? Has it ever occurred to you that you could be executed for that? Me either. What kind of a regime does that? A horribly abusive one, obviously, but more to the point, one that keeps very close tabs on its people, leading us neatly to the second thing.
After the Berlin Wall went down and people got access to the secret police files, they found out that in the worst regimes - East Germany and Romania are particularly notable here - something like one person in four was reporting to the secret police. Applied here, that would mean about 15 of our followers were spying on the rest of us, and one or two of your Mad Genius hosts was doing the same (technically, one and three-quarters, but I don't see any volunteers for 3/4 of a person). More than three people in your family? Odds are, one of them is working for the secret police.
Since I'd be prepared to guarantee that very few people here have lived in that kind of environment, I think it's safe to say most of us don't have the mental toolset to understand the kind of environment a tyranny always begets. We're all accustomed to a relatively free and open society where you don't face execution for bitching about the cost of food.
How does that kind of situation develop? It's remarkably simple. In every tyranny, regardless of ideology, the way to ensure your safety and by extension that of your loved ones, is to be a Loyal Citizen. If a theology, you go to church and make sure you're the first in, the last out, and you know every hymn and your scripture - and never, ever contradict the priests (even, or perhaps especially when they contradict scripture). If you're in Onepartystan, you join the Party, go to every meeting, and so forth. But the easiest, quickest, and apparently least costly way to show your loyalty is to offer to pass information on to the authorities.
So, you do that. It's only little things, you think, and it's not going to make you a bad person. And at first, that's how it works. You might mention the co-worker you detest complaining about the cost of food, or the crazy old guy down the street who's muttering "Dear Leader my ass", but it's not like they're actually traitors or anything, and nothing bad is going to happen. Then the person you report to drops a few hints that make it obvious you're being watched as well. So when Grandma mentions that these newfangled collective farmy things are a waste of time and she never went hungry before they did this, you feel like you have to tell them, in case someone else has already passed it on and you'll be in trouble if you don't say anything. You try to downplay it, but you still tell them. And some time later, you go to visit Grandma, and she's not there, and the young family who live there now don't have any idea who used to live there - they're just happy they have a home.
You tell yourself Grandma is in a nice retirement village somewhere, but you know, deep down, that she isn't. And that you, personally, probably turned her in as a malcontent, or a traitor, or whatever the official buzzword is.
This is why people don't rise up and overthrow the Evil Overlord unless an extraordinary event triggers some kind of spontaneous outpouring of repressed rage, grief and guilt. The Evil Overlord already has them trapped in chains they've made themselves, link by little link. If you're writing a story where overthrowing the Evil Overlord is involved, you need to remember that your characters can't chat about it in the local pub. Anyone could be a spy for the Overlord. They can't even trust their own group - remember that one in four ratio? The worse the Evil Overlord is, the more likely it is he's got that many people spying for him.
That doesn't make rebellion impossible - but it makes it a whole lot harder than Lord of the Rings and its multifarious imitators (or, for that matter Star Wars and any number of space operas) would have you think. The people you're trying to liberate don't trust you because they can damn near guarantee that anyone they don't know is spying for the Overlord (why else would a stranger come here?), and you can't trust them because you know some of them are spying for the Overlord. They're going to be suspicious, unfriendly, and arrange unpleasant, deniable "accidents" for you, especially if they suspect that they're likely to be on the wrong end of a purge because of you.
They're not going to welcome their new freedom when you win, either - after as little as a generation of tyranny, people mostly expect that the new overlord will be just like the old one, maybe with different words to wrap around the same basic reality. More often than not, they're right. We're wired to follow someone who leads. It's what kept little tribelets of our ancestors alive in a dangerous world.
The fact that good leaders are a whole lot rarer than bad ones and freedom is a lot of work is something that adds depth to your writing and world-building when it's there - and if anyone doubts this, I strongly recommend a closer look at Sarah's Darkship Thieves. Athena's paranoia is perfectly justifiable and sane in the world she's raised in - and her Daddy Dearest is very much an Evil Overlord. Yes, she is the Evil Overlord's beautiful daughter. And yes, she does manage to more or less kick off rebellion. It's nowhere near as easy as George Lucas would have you think, either.
Any other examples? Thoughts? "Someone shut that madwoman up" messages?