I think that you should get someone's consent if you're using their likeness regardless of the media. That goes for anything. They're a real person after all.

More personally, I find it uncomfortable and alien. Why? Why do that? I don't understand. It seems invasive.
I'm used to making up my own nonsense though, so I've always been disconnected conceptually from this sort of thing. I'm sorry I don't have a better answer.