I go straight from script to sketching a page layout. I find when I'm writing, the last thing I want to be thinking of is what line of dialogue goes in what panel. It kills my flow.
Deciding on a page layout is something which does take ages, but I'm going to try working vertically for a while to see if that helps. I'll deal with re-arranging it for print when I reach that stage.
If you're working with an artist, and you have a particular preference for what dialogue goes in which panel, it's helpful to have that in the script. If I were working with a writer, I'd expect that. (Unless they were comfortable allowing me the creative control to determine what goes where, and on what page.)
Here's a brief exerpt of the vert start of my comic script, when I was trying to format it properly. I use a program called Scrivner to write, which has a comic script format inbuilt. It's not free, but was well worth the money to me.
PAGE 1
PANEL 1
A ship is flying through space.
PANEL 2
A hand flicks a switch labelled ‘COMS’. There’s a glass of whiskey beside it.
SCRUFFY COURIER
This is the courier vessel 5UNFL0R, now leaving ________ space. ETA to the ______ facility, 20 Standard Hours. Pleasure doin’ business with ye.
RADIO
Affirmative. ETA conveyed. _______ out.
PANEL 3
A cigarette is lit.
PANEL 4
A pair of feet on a dashboard covered in switches, screens and lights.