All too often, books adapted into films flounder because they are too faithful to the source material.
Two films this week are examples of why it is often much better to start with a clean slate, with no obligation to make book fans happy.
Hollywood's infatuation with cute teen grumps continues in Carrie Pilby (PG13, 98 minutes, opens tomorrow, 2.5/5 stars). It's based on the best-selling 2010 chick-lit novel in which the teen character of the title hates phoneys and considers herself too smart to put up with humanity's mediocrity, especially in morals.
She is, in other words, a Holden Caulfield from J.D. Salinger's The Catcher In The Rye, a point that director Susan Johnson underlines by having Carrie (British actress Bel Powley) clutch a copy of Salinger's Franny And Zooey.
Caulfields are a dime a dozen in fiction and in movies because bitter, articulate protagonists are easy to write - they not only instigate the drama around them, but they can also describe the damage done in interesting ways.
Johnson, who crowd-funded the making of this film, works with a screenplay that tries hard to externalise the book's internal monologues, with Carrie's voiceover helping when necessary.
Carrie's neuroses and melodramatic bent drag in her long-suffering dad (Gabriel Byrne), who sets her up in a job. She meets wacky co-workers Tara and Douglas (Vanessa Bayer and Desmin Borges).
Comic actor Nathan Lane is an actor that should always be turned up to 11, but here, he opts for a fine, if frustratingly, low-key performance as Dr Petrov, the analyst who persuades the hyper-intelligent Carrie to perform five tasks on a list designed to make her less mopey, which she carries out dutifully.
Johnson searches in vain for a tone she can live with. What begins as a Woody Allen-style satire of a narcissistic New Yorker lurches into a serious story about a teen navigating the shoals of sexuality, before it stumbles into feel-good romance. Goofy pals Tara and Doug appear randomly to say goofy things to lighten the mood.
Powley is a good actress, but she plays a protagonist whose motivations flip-flop too conveniently, such that by the end, it is hard not to feel cheated.
That feeling of having the carpet pulled from under you (and not in a good way) continues in Christian drama The Shack (PG13, 133 minutes, opens tomorrow, 1.5/5 stars).
Like Carrie, this is also based on a bestseller - in this case, the 2007 novel of the same title by William P. Young. This book, however, carries a more overt fantasy setting.
Mack (Sam Worthington) is a father whose shaky faith changes into contempt for God when his young daughter Missy (Amelie Eve) is abducted and murdered by a serial killer. His bitterness threatens to destroy his family, until he receives a note that leads him back to the birthplace of his agony - the shack where his daughter was killed.
It is one thing to make a faith-based drama, but another to make one that is a form of the Santa Claus fantasy. Its makers imagine that people of faith have a wish to see the Christian Trinity make an appearance on Earth to take care of their personal suffering.
The three are represented here by actors Octavia Spencer, Aviv Alush and Sumire.
Mack is inducted into a kind of celestial healing centre in the woods where divine beings perform miracles and answer questions about the purpose of human suffering and the problem of evil in the world, without actually saying anything that those who have heard a few sermons do not already know.
British director Stuart Hazeldine cranks up the digital effects to give a fairly interesting interpretation of heaven (think flowery meadows and distant mountains), but they cannot make up for how this is a sermon, turned into a play, dressed up with computer-aided show-and-tell.
