I think this is one of those things we can chalk up to differences in media. In the book, of course, we have an omnicient narrator who can tell us the shift in Mr. Collins intentions, and of Lizzy's dawning realization of them. The film has no such luxury, so these ideas must be introduced through the action. Thus, we have the camera (as Mr. Collins) looking over the Bennet girls, and Lizzy overhears her mother speaking of the prospect at Netherfield. I think Davies does a good job of incorporating these two narrative points into the onscreen action.