Film feeds hungrily on literary texts: novels, short stories, biographies, memoirs, newspaper columns and articles — and comic books. In the way that cannibals eat the brains of their enemies to make themselves stronger, film does the same, seemingly oddly reluctant to trust original, invented narratives; seemingly uneasy about allowing its art form the freedom of its own unique integrity. Consequently film borrows the majority of its narratives from elsewhere — narratives that have already proved themselves in various ways. As a rough calculation I would estimate that three quarters of the films made in any given year are adapted from another literary source.

My own experience is typical. Of the 20 produced films, short films and television series based on my screenplays, 14 are adaptations. I have adapted novels by Evelyn Waugh (Scoop, the Sword of Honour trilogy), Mario Vargas Llosa (Aunt Julia and the Scriptwriter) and Joyce Cary (Mister Johnson). Not to mention adapting the lives of Charlie Chaplin and William Shakespeare and five of my own novels and three short stories. Adaptation is far and away the dominant form — original screenplays are inevitably the poor cousin, it appears.

And yet adaptation, for all its popularity, is almost always aesthetically unsatisfying — for several simple reasons. A fascinating new book, The Art of Screen Adaptation, goes some way to demonstrating why. The book is a series of interviews conducted and edited by Alistair Owen — the acknowledged doyen of the Q&A — with a dozen eminent screenwriters, all of whom have several or many adaptations to their names. They include the writers Hossein Amini, Andrew Davies, Lucinda Coxon, David Hare, David Nicholls, Christopher Hampton and Deborah Moggach, all renowned for the films and series they have written.

I would estimate that three quarters of the films made in any given year are adapted from another literary source.

The collective insight and wisdom gathered here reveals the small herd of elephants in the adaptation room. The first elephant is length. In any screenplay the rule of thumb is that one page equals one minute of screen time. In my experience it’s somewhat more than a minute, but this is the universal template. Therefore, a long film, say three hours, would require a 180-page screenplay. It’s an elementary calculation to realize that most novels — even average-sized novels of 350 pages —won’t fit into the normal running times of films.

Daniel Craig in Sword of Honour, a 2001 film that William Boyd adapted from the Evelyn Waugh trilogy.

As a result, most screen adaptations are in fact savage redactions of the novels or books they are based on. My Sword of Honour adaptation turned three medium-sized novels into three hours of television. A lot is necessarily missing. I would say that in the average adaptation at least 50 percent of the adapted book never makes it to the screen. Already we are in a world of huge compromises. But there are more radical accommodations and impossibilities up ahead.

Elephant number two: this may seem a banal observation, but film is, fundamentally, photography. There is one overriding, omnipresent point of view in any film you watch and that is the camera lens. You, the member of the audience, are always on the outside looking on, and this has a huge effect on what you are seeing and understanding. Film is immutably objective. This is great for setting a scene — the view of the Venice Lido, the cathedral of Notre Dame or the battlefield of Waterloo — but it’s wholly unsuitable for the inner lives, the mental anguish or bliss, the intellectual stresses and strains, exultations or enervations of the characters being depicted.

By dramatic contrast, the novel is effortlessly subjective. Whole chapters can be devoted to an hour of internal monologue, of nuances of personal, secret psychological debate. This crucial, vital aspect of the novel is virtually impossible to film. Moreover, the tools that the screenwriter has to hand to try to replicate, say, Anna Karenina’s torments as she contemplates suicide over Vronsky’s affairs are crude.

Most screen adaptations are in fact savage redactions of the novels or books they are based on.

There is good acting: but the best acting in the world cannot duplicate the incredible subtlety of a single paragraph of internal thought as described in a novel. There is voice-over: heavily resorted to in order to render subjective reflection, the cinematic version of first-person singular. But the device is somehow uncinematic — telling not showing — and too much voice-over looks like an admission of defeat. Objectivity always wins. Also, the camera can be used subjectively as a character’s point of view —but, again, too much of this can become tiresome. The fact is that, when it comes to adapting a novel, film simply cannot come close to reproducing the novel’s enduring, alluring power: that miraculous ability to enter the minds of other people.

Elephant number three: given all these parameters and compromises and sheer impossibilities, why would anyone want to adapt a complex, sophisticated work of literary fiction as a film? Why not write an original screenplay that plays to the strengths of the medium you have chosen to work in? Strengths that include effortless mise-en-scène, thrilling action sequences, huge close-ups, characters whose emotions and motivations are swiftly recognized and add to the compelling nature of a plot: a plot, moreover, that has been tailored precisely to the envisioned time span of the film — 90 minutes, two hours, three hours — and not a narrative that has been brutally butchered to fit the demanded length.

Tune in Tomorrow, Boyd’s adaptation of a Mario Vargas Llosa novel, Aunt Julia and the Scriptwriter, starred Barbara Hershey, Peter Falk, and Keanu Reeves.

As a thought experiment, draw up a list of your ten favorite films. I would be surprised if the majority were not based on original screenplays. It’s certainly true of my top ten. Film — and I use the term as a catch-all to include television, or indeed anything shot with any kind of camera — is a wonderful, powerful art form, but most of the time, 75 percent of the time, it is trying to be, or to mimic as best it can, something else: a novel or a short story, or a biography or an investigative newspaper article, and so forth.

Why not write an original screenplay that plays to the strengths of the medium you have chosen to work in?

Of course there are many successful adaptations — as long as you don’t refer back to the original. Vladimir Nabokov, who did not like Stanley Kubrick’s film of his novel Lolita, knowingly faint-praised it by saying that adaptations should be “vivacious variants” of the original source and that this is what Kubrick had achieved. However, I think Nabokov’s veiled derogation hit on a truth. If there is an “art” of screen adaptation, then it is to achieve and create an adapted film that succeeds on its own cinematic terms, reflecting the potential of the new art form it has been transposed to, and not as a faithful, diligent copy.

When I was asked to write an adaptation of Llosa’s superb novel Aunt Julia and the Scriptwriter — which happens to be set in Lima, Peru — the implacable injunction I received at the very outset was that this film was absolutely not to take place in Lima, Peru, but had to be set in the US. What seemed like crass commercialization was in fact a liberation in the end. I placed all the action of the film in New Orleans instead — as polyglot and exotic a locale as I could find in America — and the film inevitably became a “vivacious variant” of the original novel. It was forced to be different.

And here is elephant number four: is the screenwriter, set the task of adapting a novel, whether a famous or forgotten or recently published novel, really an artist? Is there an “art” to adaptation? As someone who has done a fair amount of adapting I have to say I suspect not — the artist is the one who has created the work you’re transforming. Adaptation is a craft, rather than an art, I believe. But craftsmen and craftswomen are not to be sniffed at. We are artisans de luxe, if you like, operating in a ruthless industrial medium that not only imposes stringent artistic constraints, but also stringent constraints of budget and ideology and temperament — you often have to work with very difficult, stupid and demanding people. The fact that, at the end of the day, a long novel has been rethought and reconceived as a good film (if you’re very lucky) is no mean achievement. We toil in an unforgiving vineyard, but sometimes the wine we manage to make can be heady.

Julie Christie in Don’t Look Now, Nicolas Roeg’s 1973 adaptation of the novel by Daphne du Maurier.

William Boyd is the author of several books and plays, including the upcoming Trio