Last weekend I saw The Ghost Writer, Roman Polanski's film based on The Ghost, a novel by Robert Harris. Not having read the book, I couldn't compare the movie to the original, but I did find it a terrific entertainment. Afterward, I wondered if I had missed something by not reading the thriller itself. But reading the book after seeing the movie would mean that I knew the basic plot and (stunning) ending. Would reading the book still be worthwhile? I'm not so sure. Conversely, if I had read the book first, would I have enjoyed the unfolding plot nearly as much? Again, I'm thinking probably not.
And what about those films I saw after enjoying the book -- everything from Wuthering Heights to The Hours to Julie and Julia? If I really love a book, then I anticipate having the pleasure of rediscovering the work, seeing actors impersonate characters I had only imagined, and finding a new point of view in a director's vision. I guess it comes down to the richness of the original work -- Jane Austen versus Ian Fleming, for example. If staying on the edge of your seat is of paramount importance (as in the Polanski film), then reading the book first might actually interfere with your enjoyment of the film.
Then there are those films that have pointed me in the direction of a book and enriched my experience of it. I read Tom Jones as a teenager right after seeing Tony Richardson's brilliant film. I still recall the great pleasure I had in inserting Albert Finney into Fielding's deliciously unhurried picaresque novel. Sometimes movies don't replace books at all, but actually can bring us back to reading.