Heh... interesting... I tend to work on an entire piece in steps, pausing only when I have to go do something else.
And I prevent "writer's block," which is that ugly lack of creativity, by doing like ten pieces at a time, all at different stages. So I will just have a sketch here, but a fill there but no effects, etc. That helps me because I deal with fatigue a lot and can't muster the ability to be creative. I just work on the stuff that doesn't require much creative thought. And this is the thing. In spite of working that way, I finish nearly everything.