Many people write fanfiction
. Generally, such writers make up the most devoted component of a series' fanbase.
Generally, yes, but not always.
Some people are fine with viewing and producing fan-made media without being overly familiar with the source material. There can be any number of reasons for this. For instance, someone could be introduced to a fandom at a time when the series in question is not readily available (most commonly seen with anime and manga). Alternatively, someone might have been attracted to the fanwork scene by only a single aspect of the original work.
For some reason
, this tends to show up more often in works that have lots of male characters. Needless to say, this can easily result in copious amounts of Fanon
and Canon Defilement