I'd have to say the visualisation stage! The part where I run through the scene in my imagination, over and over, before writing down the dialogue into a script. Then I do it again when it comes to drafting - I replay that little fragment and break down the scene into visual moments, which I lay out over an imaginary page before actually drawing anything. The best part is, I can do this anywhere. In the car, in the shower, while listening to work presentations... 
As far as actual 'pick-up-a-pencil' parts go though, it'd be colouring. Always has been. 