The first thing you notice is that C-3PO would easily pass the Turing Test: in a conversation, he’d be hard to distinguish from a normal humanoid. But they’re so much more than that. The droids are conscious. Speaking of R2-D2, Luke says that he’s “never seen such devotion in a droid.” The concept of “devotion” implies choice. A droid who is “devoted” to a task, or a person, is choosing loyalty over abandonment. And choice implies free will. Around that same time, C-3PO begs an annoyed Luke not to “deactivate” him. Deactivation is clearly seen by C-3PO as something to be feared, like death. Which means that droids both understand their own mortality and experience emotions, too.
They also have their own theology. When C-3PO is lowered into an oil bath to repair his joints, he exclaims, “Thank the Maker!” It’s one of only two times in the series that a character references theism. We’ll get to the other instance in a moment, but it’s instructive that in both cases, it’s a droid, not a humanoid, who refers to a supreme being.
Free will, emotions, and their own elementary religious system? We’re way off the AI scale now. C-3PO isn’t just intelligent. He’s conscious. He’s sentient. He’s a person.
And he’s a slave.