Like listening to the radio, reading a novel gives scope for one's imagination to 'fill out' the characters visually. Thus one is contributing to the work personally, and that makes it engaging. The trouble with a film of a much-loved novel is that I've already imagined how, say, Mr Darcy looks and sounds. To find Colin Firth striding about the place rather shakes my original idea of him. The strange thing about the Harry Potter films is that for me the actors exactly mirror how I represented them in my head!