It is my contention that books feed the imagination more than films. With books we essentially use just the one sense: sight; and with it we usually see only words which we use our imagination to interpret into entire worlds. Possibly the most powerful act the imagination can perform. Films, on the other hand, present us with these worlds already made manifest into vision and sound, often punctuated by particular music and shots used to elicit specific emotions in the audience. The amount of work our imagination does is, in comparison to books, quite minimal: we use it to believe that this world is real rather than a flat image, and we use it to fill in the rest of our senses.
But the remaining senses, touch, taste, smell, bear little relevance to the world of film. As a disjointed viewer, the audience is not a player in this world, and is kept apart. Touch and taste as senses are far too close for the usual film-going experience. Which leaves us with smell. This can be tricky to convey on screen as we only have sight and sound to be told of the smell. We are given then either of the physical reaction of an actor to the smell, or an actor stating the presence of the smell. (The same applies, of course, to both taste and touch but the experience is made impersonal to the audience and so has less impact. Smell however permeates and the audience could imagine that as a viewer present, they too can smell it.)
As an example of the impact that smell can make, I would like to make reference to an episode of the Simpsons. Marge has had installed a complete refurbishment to the interior of their house which is a fully computer-controlled environment. When the CPU sprays some air freshener, she says, "That really covers the cat crap." In the context of the Simpsons, that's a really funny statement, as you then imagine that the smell of cat crap has filled their house every time you've ever seen it, but you didn't know it was there because the idea of the smell was never conveyed to you. But even in books, smell is the most difficult sense to convey. As the brilliant author Aidan Chambers said in reference to his book Breaktime (first published by The Bodley Head in 1978, and without any doubts definitely worth reading):
I said, "Now you're going to ask me to get rid of the sex scene, aren't you?" She said, "On the contrary, I love it. But I want you to remove one sentence." I said, "One sentence? There's a whole bloody scene there! Why one sentence?" She said, "Well it's not in the sex scene; it's in the scene where the boy masturbates." And I said, "What is the sentence?" She said, "I want you to remove the sentence where the boy talks about the smell." And I said, "Why? Why does that matter?" There's a whole scene where he does this then thought to be disgraceful act. She said, "Smell is the one thing that causes offence to people, and we will get the book banned if it's in. They... will just refuse to buy it. Get rid of the 'smell' sentence, and it'll be alright." And she was quite right. And that's great editing, amazing editing. And she's been my editor ever since. She's a wonderful editor.
(Extract from an interview with the literary group In brief... on 28 September 1995)
All of which is a fascinating discussion, I'm sure, but what relevance does this bear on Se7en? Well, the film has many good points: an amazing plot, fine acting, atmospheric music; but it has an uncommon impact in particular because its strength lies in its use of imagination. The worst things in the film are all left unseen. You don't actually see a large amount of gore considering the gruesome methods of execution. The nastiest things are all in your imagination, and in fact the more vivid and dark your imagination is, then the more impact the film has. And that's something special.