I don't know if all writers do this--probably not--but I tend to cry a lot while I write. If a scene is even the least bit poignant, I'm dripping tears while I type. I try and write fraught scenes at home. Otherwise, total strangers stop and ask if I'm okay.
I just spent the last two hours weeping, blowing my nose, and typing furiously. I didn't even realize going in that this particular scene was important--but suddenly the book's themes of betrayal and trust were RIGHT THERE and of course I took advantage of it. But since I'm only about halfway through the book (or less), it was only a short, sharp taste of what's to come later.
I guess I'd better lay in a supply of kleenex.
Edit: I should point out, incidentally, that all this weeping has nothing to do with my personal emotions. I feel fine. I think I just have to concentrate my main character's emotions to a really ridiculous degree in order to write about them so that readers feel what the character feels.