Good point about the stomach transformation but there was also a delay that time (he had time to take in the scenery of the other bodies in there) so the transformation might be delayed if he is in a dark location?
I think the sunlight condition exists and will come into play. He is in the middle of a forest with limited sunlight and wants to transform to fight the female titan. The flashback to the well experiment makes more sense if it's forshadowing a impending failed transformation otherwise switching from a chase scene with scouting legion soldiers dying left and right to a extended flashback of an experiment from before they left somewhat kills the dramatic tension.
|