Natalie Haynes: We need to talk about screenwriters. And why films and books are different

The thing is...

The thing is that turning a good book into a good film is much harder than it looks, because the things that make a book good often don't translate on to the big screen at all. There are exceptions to the rule, of course: To Kill A Mocking Bird, The Big Sleep, Misery, and We Need to Talk About Kevin, which opens in cinemas this Friday.

Lynne Ramsay's adaptation of Lionel Shriver's award-winning novel is impressive precisely because it keeps the bits of the book which work on film (the plot, the characters), ditches the stuff which wouldn't (the narrative devices), and then adds things a film needs to look good (a red motif that makes it impossible to look at something as innocent as tomatoes without sensing blood).

But all too often books that seem perfect for adaptation don't work at all, because the screenplay can't crawl out from under the shadow of the book. The writer or director is so in love with it that he can't work out what to leave on the page. Critics of the Harry Potter films almost invariably complain they're too long and they are. The writers are too afraid to bin favourite moments from the books, and so the films are weighed down by too much detail.

Imagine if Ridley Scott had looked at Philip K Dick's novel, Do Androids Dream of Electric Sheep? and suggested to his screenwriters that maybe they should keep more of the pet goat stuff in the mix. I doubt Blade Runner would have become quite such a seminal sci-fi movie if he had, though I suppose it might have caught some of the audience from One Man and His Dog.

Films are also at risk from audience expectations when they're adapted from popular books. When Daniel Craig appears as Mikael Blomqvist in The Girl With The Dragon Tattoo, he's up against the fact that the 25 million or so of the book's readers already have a version of Blomqvist in their heads, and they may not be expecting him to look like James Bond.

Sometimes that doesn't matter: Dan Brown mentioned in the book of The Da Vinci Code that Robert Langdon looked like Harrison Ford, but he still ended up being played by Tom Hanks in the movie. Harrison Ford must have fired his agent that day. Or given him a rise, depending on how much you like the film.

Ultimately, a good film needs to stand on its own, and its screenwriters need some latitude. After all, they might write something which is just as good as the novel, or even better: Ian Fleming's children's book, Chitty Chitty Bang Bang, is long forgotten, while the film (scripted by Roald Dahl) remains a classic.