It never fails: an alien comes to Earth to study us, or spy on us, or hide from other aliens. And he/she always goes native. Just once, I'd like to see an unassimilated alien.
You might have noticed this trope in the recent Day The Earth Stood Still meh-fest, when Keanu/Klaatu goes to the McDonald's to hang out with a fellow alien, who's been hiding out here for decades trying to decide whether we suck or blow. And doggone it, the old guy has totally lost his alien perspective. He loves McDonald's, for one thing. For another, he's seen enough movies to pick up that thing where he speaks a foreign language, until he's saying something really important — and then he switches to English.
Okay, sure, he still recommends that Klaatu sterilize the Earth. But he also encourages Klaatu to try those crispy apple pies first.
But he's not the worst sell-out. The worst is the Doctor from Doctor Who, who's totally gone human the first time we see him, in 1963's "An Unearthly Child." He's ditched his Gallifreyan tunic, with the attractive posture collar, for some pastiche of an English gentleman outfit. (I totally blame the lack of the collar for William Hartnell's bad posture.) But the worst comes in the early 1970s, when the Doctor spends a few years stuck on Earth and starts actually driving a car and eating at Wimpy's Burgers. (In the novelization of "Invasion Of The Dinosaurs." I think.) Did you ever eat a Wimpy Burger? They're a great example of truth in advertising.
Commenter Hamslicer points out that I somehow missed the aliens in Third Rock From The Sun, who get so assimilated they not only eat our junk food, they also get obsessed with our human innovations, like shampoo and conditioner in the same bottle.
Rule #1: You can always tell when an alien has gone human. They start eating our crappy fast food.
Sometimes they even forget their true nature, like Beth in that one Torchwood episode. She's an alien sent here to gather intelligence for an invasion, but she believes she's human — despite the huge spikes and alien tech that pop out of her arm like a really bad rash.
That's also sort of the premise of The Stranded, a Virgin Comics title that's (supposedly) being developed as a Sci Fi Channel series. Five amazingly stylish people realize that their entire childhoods are a lie, because they're Sleepers, aliens who were stranded here on Earth, with secret superpowers and stuff. (I think the Sci Fi version may be dead, which is too bad, because both the comic and the TV pilot were written by the great Mike Carey.)
Rule #2: Aliens who forget they're aliens always have amazing superpowers, and pretty great hair.
And then there are the aliens who remember they're aliens, and even keep sight of their reason for being here... but they just get obsessed with human trivia. Like Bridwell, from Astro City #5. He's been sent to Earth to decide whether his matriarchal insect-people race, the Enelsians, should invade. And he gets caught up in our trivial dramas, and obsessed with our petty vices, especially boastfulness. It's unfortunate he happens to live in the same building as the braggart superhero Crackerjack. But even Crackerjack's habit of taking too much credit for his exploits isn't enough to make Bridwell turn against humanity — it's a group of gossipy old ladies boasting that they always knew who Crackerjack was, that finally makes Bridwell decide to wipe us out. Humans!
Rule #3: Aliens who spend too much time here always get a little too close to our bad side.
Let's face it, the best kind of aliens living secretly on Earth are the ones who take over, without any of this "going soft" business. Like the aliens in They Live, who finally use our mass media and pop culture for the proper purpose — controlling us and turning us into brainwashed slaves. (But that means they have to work in the television industry, which is a kind of punishment.) Ditto for those aliens in the story "Four Eyes" in the anthology The Nightmare Room.
Of course, even when sneaky aliens manage to take over the Earth without losing their edge or getting distracted by all our shiny nice human culture, they still end up taking on our foibles and obsessions — like the alien parasite/symbiote creatures in Stephenie Meyer's The Host. (By then, of course, they're not secret any more.)
Rule #4: You can't even rule humans in secret without becoming sorta human.
The manga and anime of Osamu Tezuka routinely feature "reformed alien spies" as supporting characters, according to this fansite.
The animated version of Ambassador Magma, features a similar theme, as Murakami Tomoko, is killed and replaced by an alien. Tomoko was the mother of Murakami Mamoru, the main human character of the series, and the alien eventually became so absorbed in the role, that she believed that she was Murakami Tomoko, so much so, that she defended her ‘family’ against attacks by her own kind, losing her life in the process.
The Irresponsible Captain Tylor features an android spy named Harumi. However, she is much more than a mere spy, for she is the alien’s best intelligence analyst, and even a saboteur. Despite her best attempts, her attempts at sabotage always fail in humorous encounters with Captain Tylor’s dumb luck. Though she is an android and should be devoid of emotions, she eventually succumbs to the title character’s kindness and charm, and decides to switch sides.
Rule #5: Aliens! Don't make your spies too cute. It'll just backfire when they decide they enjoy our human fussing, and our fancy Earth ribbons.
Additional reporting by Katharine Duckett.