I always write the script first. I have an overall plan for the comic but when it comes to the individual strip I sit and think how I can make this moment/joke fit into the overall arc with out harming it or derailing the story too much… though, at times I will take small detours if they are funny enough that they strengthen the story and have the potential to find their way back to the main plot. What fun would this be if it was JUST a straight lined planned script? Even I need some spice in my comic to keep it alive for me.
After the script is over, I do really really bad chicken scratched drawings (that don’t even vaguely resemble the characters) that work just like a story board for a movie. I can check that the motions are gonna work appropriately for the comic in hand. At this step I also make sure the text will fit with the figured with out too much overlap… I hate spending TOO much time on art then finding it gets covered up by text. It happens all the time, but I try to avoid that.
I don’t see the scene like a movie, I see more in beats; What poses/characters are needed to compliment the joke at a pace that will hit the punchline accurately.