Character development, my friend.
Glukkons went from babbling morons to Italian-mobster-accented, distinguished indaviduals.
Mudokons went from all being lip-stitched to having individuality.
Abe went from a high-voiced, lispy skinny guy with no teeth to a lower-and-more-nasely-voiced, dentally-intact character.
Fuzzles went from clean haired to messy, unkempt and even more volitile.
Characters develop for the better or worse over time.
