The script, usually. I have the overarching plot in my head, and I can usually get the visual aspects - character design, environment, etc. - down on paper pretty easily, but my mind just sort of stutters over the script.
It's a matter of finding each characters' voice - what would they say, as opposed to what they wouldn't? How would they say it? Would they phrase it in a certain way? And would they let people interrupt them without being angry? Would they get into arguments easily? Would they lie? Also, how will this particular conversation carry the plot forward? Is it just filler, or is it actually useful?
After the first twenty pages, I'll have a pretty good grasp on the characters - except those who have yet to be introduced, of course - and so it becomes easier to write the script. I just have to sit down, write out the script and then edit it into oblivion until I'm satisfied.