In Disney's The Little Mermaid, Ariel's tail is literally torn in half as she is changed into a human. The facts that it occurs in silhouette and the two ragged halves become shapely legs almost instantly does little to ease the horror of the scene. And of course, Disney being what it is, this is actually toned down from the original fairy tale. Not only was the transformation excruciating, but afterwards she felt like she was walking on knives the entire time she was transformed.
Frozen: When Elsa accidentally freezes Anna's heart, she starts to freeze from the inside out. She is shown multiple times to be in pain, as well as feeling cold even in front of a fire.
•In The Avengers, the Hulk's first transformation looks excruciating and takes a long time, because Banner is fighting it the whole time. The second time, he transforms intentionally and it only takes one smooth second, with no thrashing or screaming.
The first time Dr. Curt Connors changes into the Lizard in The Amazing Spider-Man, he is not having a good time.
In Captain America: The First Avenger, two of these occur. • Firstly, Johann Schmidt: he is shown injecting himself with the unfinished Super Serum in a flashback — fire then burns around him and it cuts to his face clearly in pain and screaming, turning into Red Skull.
Secondly, Steve Rogers. During Project Rebirth, he has the serum and Vita-Rays pumped through his body — about half way through the procedure it becomes a lot more intense and he is heard screaming from within the pod. The scientists immediately get up to shut down the machine — but Steve yells at them to keep going despite the pain (becoming a freaking man mountain when it's done).