Derivatives are particularly difficult, that's for sure.
In the software world we have licenses that explicitly lay this out.
The music industry does actually have this sorted out, with ways to license music for covers (mechanical licenses) or remixing -- although the latter is definitely complicated [1] and is similar to fan fiction in that many artists/DJs get started by making remixes.
There's an entire wikipedia page on legal issues relating to fan fiction [2].
It's definitely not an obvious thing. Using your example, how do you decide if 50 Shades of Grey is a legitimate new work or a rip-off of the next part of the story Twilight was going to tell? If a similar story was released by the author(s) of Twilight would it be considered a rip-off of 50 Shades of Grey? What if it was revealed to be in the works prior to the release of 50 Shades of Grey?
In the software world we have licenses that explicitly lay this out.
The music industry does actually have this sorted out, with ways to license music for covers (mechanical licenses) or remixing -- although the latter is definitely complicated [1] and is similar to fan fiction in that many artists/DJs get started by making remixes.
There's an entire wikipedia page on legal issues relating to fan fiction [2].
It's definitely not an obvious thing. Using your example, how do you decide if 50 Shades of Grey is a legitimate new work or a rip-off of the next part of the story Twilight was going to tell? If a similar story was released by the author(s) of Twilight would it be considered a rip-off of 50 Shades of Grey? What if it was revealed to be in the works prior to the release of 50 Shades of Grey?
[1] http://djtechtools.com/2012/03/25/legal-concerns-for-digital... [2] https://en.wikipedia.org/wiki/Legal_issues_with_fan_fiction