There tends to be some ad-libbing in pretty much any vocal work, whether 
it's radio, television or dubbing anime. At times, the scripts would be 
deliberately unspecific (i.e. "Next 20 seconds: crying", Or "Asuka walks 
away, complaining, the word 'Idiot' can be heard"). Of course, the 
actors still have to stick to the mouth movements. Perhaps this is why 
dialog seems more natural when the face of the character talking is off 
screen.

And the snapshot in the intro sequence? I don't think all of the intro 
"flashes" are taken from the series. Many of them look more to be 
concept sketches than anything else.
-- 
Defragged
http://defragged.blogspot.com