Human Aliens

Terran characters discuss the difficulty of spying within an alien society. Their agents can penetrate the human population of Avalon but cannot learn anything that is not already public knowledge. Human Avalonians no longer think, talk or even walk like Imperials and imitating them is unfeasible.
-copied from here.

Karlsarm, a Freehold outbacker, experiences the same problem from the other side:

"Sometimes he thought that humans from the inner Empire were harder to fathom than most nonhumans. Being of the same species, talking much the same language, they ought to react in the same ways as your own people. And they didn't. Their very facial expressions, a frown, a smile, were subtly foreign.
"Ridenour, for immediate example, was courteous, genial, even helpful: but entirely on the surface. He showed nothing of his real self."
-Poul Anderson, "Outpost of Empire" IN Anderson, Captain Flandry: Defender Of The Terran Empire (Riverdale, NY, 2010), pp. 1-72 AT p. 37.

But even on one planet, Earth, human beings differ so much that they might be members of different species. Noah Arkwright elegantly argues that other intelligent species are so alien that they cannot show us their inner selves. See also Noah Arkwright II.