Indeed, if we adapt a work privately without making it public, the author will never know that this adaptation exists. But Copyright, theoretically, does forbid adaptation of a work without the prior consent of its author, no matter whether it is distributed or not.
JEDIT: it was already discussed here: https://originaltrilogy.com/post/id/1108834/action/topic#1108834:
Exclusive rights
There are six basic rights protected by copyright. The owner of copyright has the exclusive right to do and to authorize others to do the following:
- To reproduce the work in copies or phonorecords;
- To prepare derivative works based upon the work;
- To distribute copies or phonorecords of the work to the public by sale or other transfer of ownership, or by rental, lease, or lending;
- To publicly perform the work, in the case of literary, musical, dramatic, and choreographic works, pantomimes, and motion pictures and other audiovisual works;
- To publicly display the work, in the case of literary, musical, dramatic, and choreographic works, pantomimes, and pictorial, graphic, or sculptural works, including the individual images of a motion picture or other audiovisual work.
- To digitally transmit sound recordings by means of digital audio transmission.
A violation of any of the exclusive rights of the copyright holder is a copyright infringement, unless fair use (or a similar affirmative defense) applies.
(…)
Making a fan-edit interferes with point 2 of “exclusive rights”, however, if you made this fan-edit for yourself and you watch it only with your family, you’d have a very strong case that this is covered by “fair use” as you’re not interfering with anyones business. (See point 4 of fair use.)