I haven't watched the show yet but at the end of the book King had an afterword where he talked about the immense amount of obsessive research he did on the Kennedy assassination literally over decades. He said he began just a few years after his death but it was still "too fresh" so he put it aside for a while but picked it up again years later. He interviewed all kinds of people, actually visited the library and places Oswald lived, etc. Obviously King's version is fiction but many of the detail is very real, which makes it even more fascinating.