A: Not necessarily. If the novel is based on the screenplay, or tells the same story, then yes, the transfer of copyright in the screenplay would prevent the writer from writing a novel, but if they're just both based on the same underlying idea, then the converse might be true. Likewise, if the novel already exists at the time of the sale of the screenplay, it can, if things are handled correctly, be treated as a separate work.
But, (and this is a big one), the real answer to the question lies in the contract for the sale of the screenplay. Purchasers of screenplays typically expect that they have exclusive rights to the story, and that they'll be able to exploit their film without competition from the writer's other works. So, the contracts contain carefully crafted warranties and representations about ownership of the material. Even if your contract doesn't specifically address author written novels based on the same underlying material, if you sell your screenplay, and then publish the same story in book form, you're looking for trouble.
You might get away with it this time, but there won't be a next time. It will likely be the last time you sell a screenplay in the small town known as Hollywood.
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