17 U.S.C 106(2) protects an author's right to control works which are derived from an original work. For example, if you create an illustration of Harry Potter that looks nothing like the movie or book character, but has a lightening bolt scar and round rimmed glasses, and he is illustrated in a scene from one of the Harry Potter novels, then you could be committing copyright infringement
based upon your derived artwork.
The counterargument is that if you are not using anything other than the "idea" of Harry Potter (i.e., a young wizard who wears glasses and has a scar on his forehead), then you are also not using any fixed expression of the idea -- and a fixed expression is what the copyright
law protects -- not the idea.
Moreover, if your illustration is done for "criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" (17 U.S.C. 107) -- and especially for parody, then that is fair use
and you are not infringing on the original author's copyright.
Unfortunately, this issue has had no direct interpretation by the federal courts. However, I believe that it's fair to say that you could be infringing someone's copyright, dependent upon how heavily you use elements described by another author.
Hope this helps.
This issue has had very little legal interpretation.