9. Thanos - The Avengers & Guardians Of The Galaxy
The Marvel Cinematic Universe's Thanos is an especially interesting case, because when the supervillain made his big-screen debut way back in The Avengers, Marvel Studios hadn't yet cast a name actor for the part, and so hired a lesser-known performer, Damion Poitier, to play the Mad Titan for his mid-credits cameo.
But Thanos' change in appearance extends far beyond a personnel shift. For Poitier's role as Thanos, he appeared via prosthetics and make-up, which were enhanced with CGI in order to exaggerate the character's facial features - namely his giant chin and bright blue eyes.
When Josh Brolin subsequently played the part in Guardians of the Galaxy, Thanos was achieved via performance capture and looked distinctly different: his skin is a decidedly lighter shade of purple, his jawline looks subtly different, his eyes aren't bright blue and, of course, he looks like Josh Brolin.
If you haven't watched The Avengers in a while, it's easy to forget just how different that version of Thanos looks to the Brolin one. It was definitely a sensible change, even if the discontinuity between the two might bother the nit-pickers.