This is basically taking the concept of parallel universes, the nature of reality and past lives to a whole new level. It takes it to the level of believing fictional characters are real. There's another theory floating around that fictional books, like Lord of the Rings or Harry Potter are not fiction at all. It says that the stories actually took place on some other planet. The authors are merely channeling those stories onto paper.

Perhaps this thinking you've described belongs to this theory I've just described. I believe the boundaries of our thinking are beginning to disintegrate, which means some really wild theories are going to come into being. Some theories end up being proven to be true, others end up forever in the trash bin.

I don't think it's helpful to encourage someone to believe they are a character in a TV show. That's only going to lead them to conflict with the actual character on the TV show. At this point, it is pure delusion. It's a way for this particular person to feel important or relevant, when perhaps they don't feel those things. In fact it may even be along the lines of someone believing they are the reincarnation of Jesus.

Therapists are stuck between a rock and hard place with this type of patient, as they need to indulge them a bit OR they risk the person shutting them out.