You need to know what people are thinking to empathize with them? Think about your statement empirically. How do you have any friends without some sort of telepathy? *lol* I am, of course, teasing here.

I rather think of secondary characters as NPCs but that doesn't mean they don't have personality or flair. When you read a novel you don't know the thoughts and motivations of every person walking down the sidewalk near the protagonists. What it should do is encourage protagonists to explore who these people are. You have to learn about who they are and what they're doing the old fashioned way, by interacting with them via your protagonists, asking them questions and watching their body language. It also encourages a person to write the secondary character in a manner they not be used to, trying to inform the audience what a person is thinking without explicitly stating such through narrative voice.

This reminds me of a PBEM I tried to run a long while ago. In an effort to expand the writing ability of my player I told them I didn't want a character sheet. I wanted a sequence of documents and pictures concerning their characters with the limitation of only one diary entry. I'd accept school report cards, newspaper articles, the afore mentioned diary entry, letters written from one relative to another concerning the character, arrest reports... just little snippets that allowed someone reading them to slowly piece together an idea of who the person was... but still leave plenty of room for surprises.

Unfortunately, they were tabletop RPG types... they just weren't artistic. Just under half of them embraced the idea.... the others just gave up before they began.