A writing friend recently asked me this, as she prepared for a talk. I thought it would be useful to other writers.

I think my best advice for writing emotion is to think of a metaphor that is unique to your character. I learned this from science fiction and fantasy, actually, where metaphors have to make sense in a completely different world. But every character is in a different world, one in
their own body. So not every character is going to feel "a tickle running down her spine" when afraid or a "sick feeling in her stomach" when she's done the wrong thing.

A girl who has been abused will go numb when she gets embarrassed. She might have a flash of memory that has nothing to do with now, or at least seems like it doesn't. She might feel hot when she should feel cold. She might feel like laughing when everyone else is crying.

The important thing for the writer to remember is that it's not just that she should have a
surprising experience of emotion, but that her emotion should feel sharply defined for her. Any page of the book about her that someone opens should feel utterly different from all the other books in the world.