While the story of Turin may work it would be fairly difficult to actually make it a PG-13 which, I assume, would be a requirement as it was for the LotR. The story is more graphic and more disturbing, what with the incest and all

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But the Sil as a whole or in part is not likely to ever be a 'successful' film, as I've spoken before because of the simple fact that it cannot possibly draw in the same fan base as did LotR. CSt spoke on the historical aspect which is very true and thusly it's not nearly as cohesive a story as the LotR and most segments. you pick are either too long or too short. Also while most, if not all of us, enjoy both he LotR and the Silm there are a lot of LotR fans that have never even read or intend to read the Silm I think this would carry over into the movie. There simply wouldn't be enough interest to make it profitable.
But if I'm ever independently wealthy I will buy the rights and make the movie myself, not for a profit, but for the enjoyment of it all