I became interested in the concept of type design for subtitles a few months ago. In my phase of cramming as much information on the subject as possible, I discovered that there is one handsdown expert on the subject--Joe Clark. Go to his site: www.joeclark.org

When you get there you can either browse the seemingly endless resources he provides, or you can email him with your question and I'll bet he can answer it.

I must say I don't really know what they should look like.
Do you have any visual examples that you could show us?

Make them hard to read and that should do it

Seriously, though, I wonder if a typerwriter font like courier would work. Subtitles were pretty primitive back then I believe.

Or are you looking for more of a nouvelle vague style for the type