A long time ago (in internet time, not galactic time), someone challenged me to write this post. Past-me gave it up as impossible, so I’m doing it instead.
The first clue is in the fact that C-3PO keeps turning up around important people. This isn’t much of a clue; after all, diplomats and politicians are the ones who use protocol droids the most and it makes sense that the same droid would be passed around among those people. But it is important to note. It means that C-3PO knows more about what’s going on in the galaxy than nearly anyone else.
He is, after all, one of only four characters to appear in every movie. Nobody saw all the events depicted, and barely anyone was in a position to even find out about all of them. But it’s a safe bet that C-3PO is one of them. Talking to the people who just happened to be the witnesses to the plot of Star Wars is literally his job as a communicator, and nobody bothers to hold their tongue around the protocol droid. It’s like how nobody notices the mailman, except more so. But don’t just take my word for it; have an example. When Anakin and Padme were secretly married, who saw it? Nobody, not even C-3PO, said or implied it was unusual for him and R2-D2 to know highly volatile secrets.
The movie makers thought of this. They didn’t want to leave someone around from the prequels who could tell Luke and the rest everything. So they inserted a line, “have the protocol droid memory-wiped.” We have no reason to think the order wasn’t carried out. But what’s missing is, they didn’t do the same to R2-D2. Maybe because he’s not shaped like a human so it doesn’t occur to them that he might know things. And who does C-3PO talk to more than anyone else? Clearly this is not an effective memory wipe.
Speaking of R2-D2, his continued presence is sort of surprising. Diplomats and senators might be likely to employ a protocol droid, but what possible use could they have for an astromech? It’d be like if a real-world dignitary hired a personal assistant and a full-time mechanic. Nevertheless, R2-D2 is always around wherever C-3PO is. The most likely explanation is that C-3PO is able to convince his employers to go slightly out of their way to use that particular droid. That’s not hugely significant, but it does require him to have more influence than he appeared to use in the movies (i.e., none).
So by the start of the original trilogy, the droids know what happened in the prequels. This is our first clue that there is definitely some plot going on. They could have simply told Luke or the Rebels about Darth Vader’s backstory. They could have at least informed Luke that they’ve seen the name Skywalker before. But they didn’t. They never once even say the word “Jedi” or let slip that they fought in the Clone Wars. This means that they are consciously, actively trying to keep a low profile even from the Rebels, even while they try to make sure they win. This also explains why C-3PO uses the voice he does. He has an artificial voice box; he could sound like James Earl Jones if he wanted to. He picks the least intimidating, most ignorable voice possible. Nobody notices the mailman, and they definitely don’t pay attention to a protocol droid.
And most importantly of all, C-3PO knew what would happen on Endor. There was no obvious reason for him to go there, but he never once asked “why am I going along.” A shiny and definitely not camouflaged protocol droid in the middle of a secret military operation? Something’s up. C-3PO as he was portraying himself to the Rebels would have liked nothing more than to not go to Endor, and it’s not like anyone would insist on it or even suggest it.
But when they got there, they found that someone had trained the Ewoks into a fighting force capable of interfering with stormtroopers. You didn’t think hunter-gatherers could do that on their own, did you? That’d take someone like a Clone War veteran or two. It couldn’t have been a member of the Alliance; they didn’t even know the Ewoks existed. But they have somehow set up an effective resistance. Some of the traps the Ewoks used were useful only against AT-STs, meaning they were built and invented recently. (The thing with all the logs rolling down the mountainside could knock over anything big, but the two tree trunks swinging down simultaneously would be useless against a predator or anything else capable of looking sideways.)
And what’s the first reaction of the Ewoks when they see C-3PO? They recognize him. What’s the alternative, that they start worshipping anything shiny? They must have seen plenty of Imperial machinery, but didn’t have that reaction then. And they didn’t react much to seeing R2-D2, which is also why we know it’s 3PO behind this particular plot and not R2. Well, that and the fact that R2-D2 doesn’t speak the language.
Also worth noting is that C-3PO tells Luke that it’s “against his programming to impersonate a deity.” This is an obvious lie. Luke had other things on his mind at the time, but the viewer has the time to notice that a droid’s programming is not analogous to ethics. If it were against 3PO’s programming, it wouldn’t be that he considered it wrong or unpleasant; it’d be impossible. He would no more be able to impersonate a deity than my calculator can tell me 2+3=6.
Finally, there is one detail hiding in plain sight throughout all of the original trilogy. Every time 3PO introduces himself, he calls himself “C-3PO, human cyborg relations.” As a droid built in a nine-year-old’s garage, he can only really be said to have one relation: Anakin Skywalker. And at the time of the original trilogy, Anakin Skywalker is indeed a human cyborg. C-3PO knew Darth Vader was Anakin Skywalker before George Lucas did. And now you can’t unsee it.
So obviously C-3PO is up to something involving both lying to the Rebels and saving their bacon. What is it? I wish I knew. One thing’s for certain, though. There’s a lot more to this droid than you see.