I'm half-and-half! I have most things scripted well before I get to drawing them, but I certainly don't have everything. And I almost always adapt and add to the script once I start storyboarding an episode. In particular, things to do with character development and relationships are figured out well in advance.
I'm actually starting to run out of the plot I have scripted - it'll certainly be all gone before December - so I need to start figuring out how to link this reflective, sombre part of the story to the next action-filled part, and choose what moments are going to bridge that gap. I also need to think up plot beats for the action part, because that's definitely not something I have much familiarity with, and I'll need to refer to some old-school plot structures to help me along. That'll be tricky.
All that said, I have the overall 'shell' of what happens complete. I know the direction my comic is going in. It's just diving into how some of those things happen which I need to get the ball rolling with, and soon.