Originally Posted by
Ielenia
We clearly see the Jailer having absolute, total control of Anduin in the cinematic where he attacks the Archon, even down to speaking through him. Anduin really feels like an extension of the Jailer at this point, with sometimes being allowed to see what he is being forced to do. For what reason, though, I can only speculate, probably to help break his mind.
While with Arthas, while he was indeed corrupted, it seems the character still had agency, as the one speaking to us was indeed Arthas, when he taunted us back during the leveling experience in Wrath, inside the Halls of Reflection, Icecrown Citadel, etc.