How Much Credit Do Adapted Screenplays Deserve?

As I’m preparing for my best of 2012 movie list, I had something quick I wanted to discuss: I can never figure out how much credit to give adapted screenplays. I understand that adapting the source material for a medium it wasn’t intended for is an artisitic endeavor in and of itself, but how much credit does a filmmaker deserve if the plot, character and themes are established before the first word of the script is written?

Tom Hooper’s Les Miserables is the poster child for this problem. While it’s rarely subtle, I love Cosette as the literal emodiment of freedom, fought for and won by the old generation (Jean Valjean) only to realize that sometimes freedom has to be incubated, fought for, then protected (Cosette’s marriage to Marius). But the movie itsefl? I could take it or leave it.

So when reviewing Les Mis, do I give credit to Hooper for his portrayal of what it takes to achieve freedom, even though I’m fairly sure it’s more prominent in the book (though, probably not the musical). Or should I just judge Hooper based on his technical skill at adapting the source material? How much credit does Hooper deserve for retelling a two-hundred year old story?

Where I’ve come down (though, I can definitely be swayed) is that if I truly view movies as separate works of art from their source material, I have to judge them as separate works of art. Sure, most of what I like about Les Miserables should be credited to Victor Hugo and not Tom Hooper, but I’m reviewing Hooper’s work of art, not Hugo’s.