Some videos I script and others I don't. Often times I'll just let an idea be very freeform and take shape as I go along, other times I had a definate vision for every part of the video, other times it'll be somewhere in between.
Soul of an Angel: Wasn't really storyboarded, was graphed. I counted # of measures and basically drew several different tracks on paper, each labelled like "fast layer" "melody layer" "refrain stills" "background" etc. Then I marked where they would start and stop. The actual imagery was not planned out, but the way they mixed together was.
Dual Opening: This needed no script because it was a shot-for-shot parody. Thus my "script" was the original Eva opening and I just found scenes to match.
The Anime Polka: No scripting here, did it all spur of the moment freeform. Came up with ideas of which series to use where first, and let it flow from there.
Cowboy Blues: This one wasn't scripted at all, because my original concept for it was so barebones that the video almost didn't happen. Once I finally got an expanded concept that would work, I listened to the whole song again and scripted it out in my brain, i.e. I listed to the song so many times that I knew exactly which scene would go where and went from that, so in a way it was scripted but not on paper.
Before I left for Japan I actually started scripting videos more. One of my AWA pro entries was heavily scripted for the whole first section, minorly for the latter two thirds although most of it was planned out.
My AWA Masters video was heavily scripted in my head ahead of time. I listened to the song on loop for like 3 days, while rewatching the series and ripping it.
However my other AWA pro entry was a "speed" video that I talked about in my journal a while back, so there was no scripting there really, and my new AWAExpo entry is a parody so there was no scripting there.
On older vids, The End of Me was scripted on paper, Life is But a Dream was scripted entirely in my head but the whole video was down shot-for-shot in my mind, since the planning stages on that one were in the neighborhood of months in length.
All my other vids not listed here involved no paper scripting. Some of them involved minor mental planning, but many just evolved as I worked because the scenes and editing just flowed from the concept and the video "made itself."
Currently since I'm in Japan and cannot edit, I've been listening to the songs I plan on doing AMVs to when I return a lot and am planning things out in my mind. I brought along the DVDs of a couple series I plan on using, so I may eventually start storyboarding them on paper here in Japan so I can get a jump start once I get back to my editing equipment, but what might happen is - like Life is But a Dream - I'll just know the whole combination so well that it won't be necessary.