Here is a little copyright intro for you. This is not legal advice; if you are in need of particular answers for your situation, please consult an attorney directly, as other commenters have indicated. Copyright protects original creative expression. This is in contrast to general ideas, which cannot be protected by copyright. The difference between ideas and expression is hard to define, but a common example is that…
It's much simpler than that actually, copying subtitles is a direct infringement on the rights of the script writer. Whether someone read the script to you over the phone and you copied it down or you took it from the movie makes no difference. No need to get into derived works.
It is a derivative work when you transcribe subtitles because you put in your own creative (and copyrightable!) elements into it. You choose how to represent sound effects, how to disentangle crosstalk, how to transcribe dialect, and so on.
So, if the movie creator decided to rip your subtitles off and sell them, you could actually sue the movie creator!
As a practical matter, it makes little difference. Most judges won't parse this finely, and you are equally liable whether under the copying or derivation right.