I just finished episode 24 of EVA, and I noticed that the last two previews I'd seen looked a lot "rougher".
The preview after episode 24 consists of a series of what seem to be rough drawings or sketchs over a voiceover.
The preview after episode 23 was animated (in that there were some "moving" frames), but looked unfinished - the colouring and lining looks incomplete. Moreover, some of the images look like they come from a manga (in terms of, for instance, speech bubbles, or hiragana/katakana given for some kanji - although it seems possible to me that the bubbles might just have been taped on so as to give people involved in production some idea of the dialogue in each scene). For example:
Comparatively, the artwork in prior episode previews seemed somewhat more "finished", and, at least in my memory, was usually animated. This I presume is probably because of the fact that EVA had production issues toward the end of the anime run.
But less obvious to me is this: what are these speech bubbles that show up in the episode preview for episode 24 actually for? Are they something that feature in preliminary anime sketches?