"Do you get ideas uncontainably leaping out at you for script n+2 while
you're working on script n?"
Constantly. And while en route to work. And while in the shower.
And...well, you get the idea.
If I'm writing script n, and something hits me, I grab the nearest thing
that isn't on fire or moving, and scribble it down, with the result that
my desk is constantly a snowstorm of bits of paper and post-its. The
really big ones get post-it'd to my monitor at the B5 stage and my home
office. By the end of this season, my monitors looked like hedgehogs....