You only need a buffer, you don't need to store the whole movie.
You do need to store the movie if it is a 24 hour rental. Maybe not NEEEED, but it is a smart thing to do to avoid bandwidth being soaked up on each review.
I looked at movies I have, and they are 2 GB for about 2 hrs. or HD. So people can rack up a nice movie night on their device and not need to deal with potential glitches and gaps due to stilted internet connections.