While most television shows will turn a blind eye to fanfic written in their universe, some book authors are not so forgiving (while some, like Flint mentioned above, are quite open to fanfic).
While the characters might be romantically involved in the source material, a het fanfic will, more than likely, spare no details of their sexual relationship.
If you write fan fiction (or fanfic for short) you should be aware of both your legal rights and the rights of the author you are deriving your work from.
Novice actors who find themselves in a 'cult hit' often have to be gently introduced to what is fanfic, to prevent culture shock.
Marysue is a much maligned Original Character (OC) that the fanfic author inserts in her story to serve as her avatar.
Main characters are often 'offed' in fanfic, if it suits the story, something you will rarely see in show 'canon'.
An erotic piece of fanfic that posits a romantic relationship between a male and female character from source.
The authors that will come after fanfic writers in their universe, of course, have legal right on their side.
An erotic piece of fanfic that posits a romantic relationship between two male characters from the source.
Strictly speaking, writing and showing fanfic to others is a violation of the copyright of the originator.