Glass uses the movie entire as libretto and, for lack of a better word, set. The singers are synching to the actors on screen, but it doesn't ever come across as mere dubbing. It truly does feel like an opera but one in which the film takes the place of the stage. The music is lush and a lot more lively and varied than a lot of Glass' other work. Glass has said that one of the drivers for doing this was to make an opera that could be easily toured to places that did not have the resources for conventional opera - all you need for Glass' La Bell et la Bete is projection and a modest space, because there are are only a 4-6 singers (they each sing multiple parts) and a small ensemble. I saw it in New London in 1995.
A little googling shows that Glass did also write an opera based on Cocteau's Orphee, but it uses the screenplay (cut down some) as the libretto and is conventionally staged. Coincidentally, it premiered in Cambridge at ART in 1993. I have not heard it, but if it's on Naxos, I will give it a listen.