A very interesting piece of meta which shows how the coherence of the show lies not in the technicality of the world JW created but rather in its mythological depths. Now are all the parallels we see in the story intentionnally there, I don't think so, but a big part of them is: after all JW's world is a world of metaphors and symbols. Parallels, mirror plays, doubles are an integral part of that writing arsenal and the writers sometimes pushed very far in that direction, if you want to look at how they used the costumes from time to time. About Spike, I doubt they had an idea of what they would do with him as soon as season 2, but what's interesting in JW's recent interview is how this character attracted his attention from the beginning (and somehow I think that's what permitted him to stay in the show rather than his single popularity) and how, as the show progressed they discovered new layers and new possibilities for him. How did they elaborated his story to give it such a coherence? Probably in several ways: planning in advance,sometimes using ideas that had previously cropped up during their work but hadn't been developped at the time being, always keeping a vey clear view of where the character was at the end of a season. In regard to Spike's development and how the writers worked, I think Restless is an important episode. There's a little piece of meta about it in my LJ if it interests you.