It's usually a big kind of vent of frustration or anger or sadness that puts me in the right frame of mind to write. It's such a cliche to say that artists write when they're down but it's true for me. It's a relief to get out what's eating away at my heart or my soul or my head.
I'm one of those people who is actually inspired by a deadline. I might not sleep for many days on end it may not be good for my health but it definitely helps.