Hi Ian,
Don't be discouraged. What you are doing is very valuable. Many people speculate about U.S. copyright law though they are not experts.
I'm not an expert either, but I work in the area of subtitles and film captions, specifically for accessibity for the Deaf and Hard of Hearing in the U.S. - 30 million people. (About as many people as the entire population of Canada!)
Regarding your efforts and copyright law, one of the main issues is whether you are aiming to be seeking any profit from your efforts. If you are just seeking to build up an index of film characters and quotes, you are probably within the Fair Use doctrine, and can carry on, as long as you indicate appropriate copyright notices on the material you post.
In this period of global cross-cultural communication, your work has many possible uses which would certainly fall within the Fair Use doctrine of copyright law, including making available information needed for making film/video accessible to Deaf in U.S., Deaf in non-English speaking cultures, and non-English language access to films/videos not otherwise subtitled or dubbed.
I just heard an interview with David Huynh, new guy at FreeBase.com, and it sure sounds like he is making interaces to browse and view specific data details exactly along the lines you mention.
You might check out some sites like:
mysubtitles.org
dotsub.com
good luck,
Greg
owner
DeafAccessFilms.com