Yes, I know for a fact that you can unwittingly serve the devil while in a state of mortal sin. I was a screenwriter and all the movies I dreamed up were definitely channeled, but I had no clue where the ideas were coming from.
As soon as I was baptized I realized I couldn't possibly make these movies and call myself a good Christian; but before that, I was deceived by my sins to think that I could.
I see fornication as sort of like a devilish generator, it's like it puts you in his service as a slave, so it is no surprise that the devil is trying to make everyone into a sex fiend. I think there is even a kind of "sex power" which is why Satanists are always having orgies and so on, it puts you MORE into the devil's service the farther you go, and then he will "elevate" you in a worldly way so that you can influence others and serve his designs.
Rock stars are also definitely channeling the devil, but they aren't all possessed. They are just receiving inspiration from the devil that in their blindness they take to be "creativity." This is not a hypothesis but fact, because I have lived it.
However, there came a time when I did know it was the devil controlling me, and had to make a choice. I don't know if others are faced with the same clear decision at a certain point, though I would assume they are since God gives us free will. So it seems that for a while it's unconscious, and then when you have stepped into the web, it becomes conscious, you feel like you've gone too far and can't get out.