I actually feel more empathy for people I don't know at all, as opposed to people I sort of know. If I know a person, but they're not my friend, I tend to start thinking of that person as an NPC, who has programed responses, acts the same way, is predictable ect. It's hard to feel empathy for people you know enough to know what they act like, but not well enough to know who they really are.
When you don't know someone at all, they're a blank slate, for all intents and purposes, and you can project.
When you don't know someone at all, they're a blank slate, for all intents and purposes, and you can project.