Under deadline. As published authors, if we're under contract with a publisher, we are constantly under deadline. Most of us don't stress out about this too much, until one day we wake up, glance at the calendar, and realize we'd better get crackin' if we want to have any hope of making that deadline.
Some of us - I'm one - sit down and calculate how many days are left until deadline and the minimum number of words we have to write per day. Since I still currently have the day job, I usually only write the bare minimum. Other writers take off work, lock themselves in a room or rent a hotel, and go on non-stop writing jags for an entire week or weekend.
For me, the level of stress is directly proportunate to how well the book is going. If the story is going along at an awesome pace, I'm happy and mostly carefree. Book will get done, I'll look for whatever concerns might be lurking int he back of my brain as I write (is the hero alpha enough? Is the heroine too passive or too agressive? You get the idea!) And I'll fix it all nice and neat before I email it in to my editor.
I have one month, one week, and a couple of days until this Pack book is due. I'm on chapter 10 of 15 or so, and well over half my required word count. I'll be fine. For whatever reason, this story is flowing smoothly. When I miss a day or two (like this past weekend), it's easy to make it up and write double the daily words. I like that.