Books make poor screenplays, writes DONALD CLARKE.

Harry Potter fans are not happy. A recent Chicago screening of the upcoming Harry Potter and the Half-Blood Princegenerated a typhoon of internet posts from disappointed acolytes. Apparently, they've changed the ending. They've inserted too much romance. They've cut an important series of "memories". As you read on you seem to hear Kathy Bates' deranged character from Miseryshrieking when she discovers an apparent anomaly in a favourite serial. "He didn't get out of the cockadoodie car!"

Elsewhere, the continuing dispute between Watchmenfans and the larger mass of film critics continues to bubble on. One post on the Internet Movie Database offers a neat summation of the main bone of contention. "This review said that the film was 'too faithful' to the graphic novel," the post reads. "How is that even a criticism?" Now, there's a question worth considering.

Since studios began adapting novels for films, readers have been complaining about a lack of faithfulness to the text. Come to think of it, I believe that certain Elizabethan graffiti suggest such complaints go back further still. "See ye Macbeth by that cockscomb Shakespeare?" one scribbling asked. "He has, by my troth, changed the ending from Holinshed's Chroniclesand the witches be not nearly so fearsome. It sucketh."

Let us put this in plain English: Faithfulness to the source material is not a virtue in itself. Hang on. That doesn't look dogmatic enough. Let's call up some italics: Faithfulness to the source material is not a virtue in itself.Got that? If significant changes in the narrative appear to weaken the film, then by all means kick up a fuss.

When Stephen Fry grafted a happy(ish) ending onto Bright Young Things, his adaptation of Evelyn Waugh's Vile Bodies, he left himself with a gelded, toothless romp. Waugh's darker ending would have made much better sense in the cinema. Other examples will, no doubt, announce themselves to the well-read cinemagoer.

Now consider Kubrick's Lolita, Tarkovsky's Stalker, Altman's The Long Goodbye, Coppola's Apocalypse Now, Whale's Frankensteinor Hitchcock's The 39 Steps. All are based on (or, at least, inspired by) admirable works of literature and all take wild liberties with the original text. Each director knew that his primary concern was to deliver a film that played well when the lights went down. If you can write a successful adaptation of Great Expectationsin which Pip has an affair with a goat, then proceed. I, for one, would like to see it.

In recent years, however, producers have become increasingly fearful of altering the slightest comma, colon or hyphen in a popular text. As a result, we have been stuck with lifeless, overstuffed exercises in cinematic embalmment such as Watchmen, Harry Potterand, worst of all, The Da Vinci Code. If Hitchcock had been asked to adapt Dan Brown's turgid potboiler, he would have kept the killer monk and thrown the rest in the bin.

It wouldn’t happen today. I blame the internet.