It’s subtle things like how Darth Vader refers to everyone by their rank, species (in the case of Chewie), or last names, even his own son. He mostly calls him “Skywalker” (until the end). Can you imagine calling your own son by his last name? It’s almost dehumanizing, showing how Vader views everyone as tools, objects. Not only that, it shows how much he has mentally disconnected himself from everyone around him. His ability to empathize with anyone (until his son, in sparse moments until his redemption) has gone cold. He’s separated himself so much from Anakin in his mind that it’s not awkward for him at all to refer to his son by just his own last name like it would be for a normal person. In his mind, he’s just Darth Vader. Anakin Skywalker doesn’t have meaning for him (or at least, he acts like it doesn’t).