No matter how much you rave about how good – or bad – an adaptation is, you can almost always expect the same response: Yes, but is it faithful to the book?Fidelity to the original, popular wisdom tells us, is all that matters. Look at the 11,000+ people on Facebook who believe that ‘Harry Potter 7 Better Be 7 Hours Long’. Adaptations should, apparently, behave towards the books they’re based on like clingy lovers, doing everything their partners do and never falling out. They should be considerate, reliable and above all else faithful. Or, to put it another way, unoriginal, uninspired and uninspiring.That’s not to say that faithfulness is in itself a bad thing. It’s the commonly-held misapprehension that faithfulness matters more than anything else that’s the problem. If this were true then adaptation would be a pointless art-form: why adapt, if you’re just treating a film like a dot-to-dot or paint-by-numbers, lazily transferring the black and white of the printed page to the moving pictures of the cinema screen?Why adapt if you accept the popular belief that the book will always be better?The truth is that just as novels and short stories can achieve things which film can’t mimic, filmmakers can do things which writers can only dream of. Every art form has its limits – and its potential. The best adaptations are those which pretend they’re not adaptations at all: those which don’t try to repeat what the book has already done, but focus on what the film can do instead.The opening sequence of The Exorcist is a case in point, nine and a half minutes in the Iraqi desert which prove that the difference between reading and seeing can, quite literally, be believing.The film vacillates obsessively between loud, rhythmic noises – the call to prayer, pickaxes on rock, hammers on anvils – and periods of near silence. And then there are the faces, a series of unsettling close-ups punctuating the wider shots. The one-eyed blacksmith. The increasingly anxious Father Merrin. And, most disturbingly of all, the barely-glimpsed, utterly malevolent visage of the old woman in the carriage which nearly kills him – so blurred and so quickly gone that we are left wondering if it was even human. The effect is lost in writing about it. But when watching the film, the presence of evil is utterly palpable; our reaction to it, visceral. All this before we have any plot – before the conventional work of adapting a book begins. The recent adaptation Children of Men succeeds for similar reasons. Unlike The Exorcist it bears no resemblance at all to the original novel, other than its use of the book’s dystopian premise of worldwide infertility. Uncomplicated by plot and character, the film hangs on a series of action set-pieces conveyed to us through unbelievably long and highly-choreographed tracking shots. It’s what the film can do, not what the book did, which matters.The filmmakers behind both films realised that whereas our response to a book is ultimately linguistic – we process the words, then respond – our response to film can be sensual and more direct. Overemphasizing the value of faithfulness to plot and character ignores what gives film its distinctive power, namely the hypnotic interweaving of sound and image which transports the viewer to another place.But adaptation, of course, is more than simply a synonym for filmmaking. It’s about grasping the power and potential of cinema, but it’s about grasping the power and potential of a book as well. Ultimately an adaptation’s success (or lack of) depends on how it uses its source material. The problem for filmmakers is that the all-important balance between faithfulness and unfaithfulness, borrowing and originality is unique to every film.Just compare Brokeback Mountain and M*A*S*H – the former so clingy in its relationship with the story on which it’s based that they could get married (or at least get a civil partnership), the latter so sluttish in its use of improvisation that it bares little resemblance to its script, let alone the original novel. That both are excellent films has both nothing and everything to do with how faithful or unfaithful they are. They’re excellent because they strike near-perfect – though completely different – balances between use of their source material and original input. Ang Lee’s unobtrusive style is exactly what Brokeback required – just as the episodic anarchy of M*A*S*H reflects the film’s focus, the madness of war.Once we get over the tendency to condemn filmmakers for the sort of creative freedom which we celebrate in authors, it’s difficult to say definitively that any book is unfilmable. Certainly, the more stylised the writing, the trickier the process of adaptation becomes. Billy Bob Thornton’s All the Pretty Horses is disappointingly flaccid compared to Cormac McCarthy’s brilliantly idiosyncratic prose style.But then Hubert Selby Jr’s equally unconventional portrayal of addiction in Requiem for a Dream has more than met its match in Darren Aronofsky’s disorienting adaptation, a film not unlikely to induce seizures – and in that respect, it does complete justice to the book.When adaptations are at their best – when they surpass the original – the book versus film debate can be settled with a simple analogy: it’s the difference between inhaling a drug and injecting it, as the characters in Aronofsky’s film find out to their peril.