Unlike writing a novel where I think it's key to keep putting down words to hit that 100k mark, writing a script requires a whole lot of sitting or walking around and rolling broken ideas over and over in your head until they finally shatter and the jagged pieces come back together in the right way.
It is guilt-inducing. You are, in fact, doing nothing while doing this. When I smoked I would go outside and stare at the tip of a cigarette while thinking and it would focus me. This is what I miss most about smoking.
I wonder why screenplays are different from novels in this. One reason might be that screenplays must be so focused, whereas novels have room to explore. It might also be a practical matter. Novels (and individual scenes in novels) take longer to write so you have more time during the process to do that thinking after you've finished your pages for the day. In essence, you're spending the same amount of time thinking about the story, but there's more writing required for a novel so your writing doesn't get ahead of your thinking as often.
