I do a very light underdrawing in pencil, if it is a paper comic.  If it is a digital comic, I have the luxury of getting to do more rough drawings.  These don’t take long, because I believe that a lot of the time, my first set of lines are the best ones, in that they capture the movement and expression I’m looking for.  If I labor on something too long, it looks bad.  But that is why my comics often get a “they look dashed off” description - I am always chasing the energy of those first lines.