If the copyright owner could not reasonably claim that your use would have a financial impact, it is more likely that a court will make a finding of Fair Use. Copyright owners are allowed to be speculative here, however. For instance, even if an author has no intention to develop his book into a screenplay, the court recognizes the economic value of his right to later choose to adapt a screenplay if he wants, so the court would still consider this to be a potentially large economic impact. A large consideration for the court is whether your work could be said to "fulfill demand for the original". If it can, your Use may not be Fair.
Click here to learn if you are protected by Fair Use