I am, again, inadvertantly competing with a couple of (bad) film versions of texts that I am teaching in my classes this semester. First, there's the mega-blockbuster Troy, starring Brad Pitt as Achilles. Now, I haven't seen Troy (though I have recently resolved to do so ASAP), but my understanding of it was that the filmmakers did not make any claims to suggest this was an exact rendering of Homer's Iliad. Nevertheless, most of my students have seen it already and their attitudes toward the text-- and, in particular, toward certain characters-- are already shaped by the film version.
I'm also working against The Human Stain (starring Nicole Kidman and Anthony Hopkins). Although that film stays pretty close to the text of Philip Roth's novel, it involves some of the worst casting in the history of film, in my humble opinion. Stunningly beautiful Nicole Kidman is cast in the role of a down-and-out working class janitor, who becomes the love interest of the main character, in large part, because of her outcast status. And the very-white and very-British Anthony Hopkins is cast in the leading role of an ambiguously-raced professor and former boxer from New Jersey. Philip Roth, as many of you know, is one of the quintessential "American" authors of the "American" experience. And The Human Stain is, even more than some of his other novels, really about the "American" experience of race, politics, class, etc. Not to sound overly-nationalistic, but the decision to cast an Australian and an Englishman in the two leading roles was a bad call.
I don't think all film versions of literature are bad. I thought the 1992 version of Of Mice and Men was fantastic, as was the 1998 Fear and Loathing in Las Vegas. I'm sure there are plenty of other fine films out there. But when you are working against a bad one, it's really tough to overcome. For one thing, it is extremely difficult for students to let go of the specific images/people presented in the screen-versions, and I think that seriously handicaps their imagination, if not also their critical capacities. (I'll have to defer to my eminently insightful friends, Professors Grady and Vaught to comment more on the relationship between thought and image.) The problem, in the end, is that if your students have already seen these films, there is little you can do to effectively "correct" those impressions.
As the old saying goes, you can't unring a bell.