Not the first time in all movies. Think of
link (my favorite) in Bruce Almighty or
link in Dogma (didn't care for her portrayal in this role [this is not because of her gender]).
As for if they will do it, I don't know but I think they might. I think I read somewhere someone talking about Kripke talking about the difficulty of casting "God." Although it would be very tricky, I think that they could do it. In my opinion, they have done a very good job of casting on this series, in general.
I recently watched the film
link, which retells the Greek legend of Orpheus and Eurydice set in 1960s Brazil during carnival. There is a scene where Orpheus goes to a
link link and has a conversation aloud with his dead beloved. It is then shown that the voice of his beloved is coming out of the mouth of a black woman next to him, who is in a trance. Perhaps God will talk "through"
link in this way.
I do not see God taking a human vessel (who could handle it??) or being an ambiguous, booming voice (done so many times before). I think if God is actually in a scene, rather than Castiel reporting "I just talked to God," that he will portrayed by an actor.