When I'm developing a story, I spend long periods in my head improvising dialogue and scenes. This is especially true when I'm too sick to sit up with the laptop. 

Sometimes the characters come into my brain and "take over." This is common phenomenon for writers, when their characters seem to take on a life of their own and your job is to watch and take notes. Other times I push them around and see how they react. This is often how I stumble across breakthroughs or important background details. 

At any rate, even when I'm flattened I can usually get some "writing" done in the form of scrawled notes to look at when I'm up again.