Movies are an interesting artform. They are probably the most emotionally-engaging or intense form- they produce perhaps stronger emotional responses than anything else; but seem limited in their depth and resonance.
The very best movies therefore are not much more than themselves - they create an image, a memory - but this residue of the movie experience does not become a symbol: that is, the movie experience does not resonate with the rest of life but remains detached and un-integrated.
Movies are hyper-real - their details and edges are hard and fixed. Great books, by contrast, are fluid, diffuse: a movie is like a fast and narrow river, a book is a tidal estuary.
In this sense, I do not think there can truly be 'a great movie' - but only a powerful movie.
For art to be 'great' requires that it has the capacity to become part of life, to flow-into life - as does a great book or piece of music or drama.
Movies are so definitely and intensely themselves that they do not seem to do this.
There is also a problem that movies usurp imagery - and this is particularly a problem when movies are 'based-on' great books; because on the one hand the movie may permanently distort the metal imagery of reading, while on the other hand the movie experience simply cannot be as important as that of a great book.
For this reason it would be better if movies stayed-away-from great books - and if people who like a particular great book would stay away from movies 'based-on' it.
In this instance, there is only one possible direction of result, and that is the movie will firstly be inferior to the great book (which perhaps does not matter much); but secondly that the movie will tend to damage the future experience of the great book in positive proportion as it is a 'good movie' - the better the movie, the more the damage (which does matter).