Faithfulness is the wrong question, and here is the better one
Faithfulness is the wrong question to ask of a book-to-film adaptation, because fidelity measures the wrong thing: it counts deviations from a text rather than asking whether the film produces the effect the book produced. The better question is what the film was trying to keep, and whether the change it made serves that thing or damages it. A film can follow a novel scene by scene and lose it completely. A film can rewrite half the plot and land it. Counting differences will never tell the 2 apart, which is why the answer to "is it like the book" is almost always useless and the answer to "what did it decide to keep" is almost always interesting.
The measure that replaces fidelity: 4 points, scored apart
The measure that replaces fidelity is a set of 4 points scored separately, because an adaptation travels a different distance on each: plot, the sequence of events; character, who these people are and what they want; voice, the way the story is told and by whom; and theme, the argument the book is making. Scoring them together produces a single number that hides everything. Scoring them apart produces a description. Ridley Scott's The Martian of 2015 sits close on plot and character, drops most of the arithmetic that fills Andy Weir's novel, and holds the theme, competence as a form of hope, exactly. That is a useful sentence. "82 per cent faithful" is not.
Scored apart, the 4 points also show how a single change can leave 3 of them standing and remove the fourth. Alfred Hitchcock's Rebecca of 1940, from Daphne du Maurier's 1938 novel, is the standing demonstration: the plot arrives almost whole and the theme does not survive the crossing.
Du Maurier's 1938 novel has Maxim shoot Rebecca. His guilt is real, and the second wife's discovery is that she has married a murderer and stays anyway.
Hitchcock's 1940 film makes the death an accident, because the Production Code would not let a murderer go unpunished. The plot survives; the moral floor of the book does not.
How faithful the Frankenstein films are, and the test that decides it
How faithful any Frankenstein film is comes down to one test: does the creature speak? Mary Shelley's 1818 novel gives him language, reading and an argument he makes at length in his own defence, and the book's whole moral weight rests on the fact that he can state his case. James Whale's 1931 film takes the language away. A mute creature cannot argue, so the film becomes a story about a monster rather than a story about a maker's responsibility, whatever else it keeps. Kenneth Branagh's 1994 version restores the speech and the Arctic frame, and the category shifts back towards tragedy. Guillermo del Toro's Netflix version returns to the articulate creature as well. Each new production is judged by readers on that single point long before anyone counts the scenes.
Where closeness genuinely is the point
Closeness genuinely is the point when the book's effect lives in its surface. Dashiell Hammett wrote The Maltese Falcon in 1930 almost entirely in dialogue and observed action, with no access to anyone's thoughts, so John Huston's 1941 film can carry it across nearly intact and lose nothing: the novel was already shot in the third person, from outside. Cormac McCarthy's 2005 No Country for Old Men works the same way, which is why the Coen brothers' 2007 film is regularly named the closest major adaptation of recent decades. Both books are, in effect, pre-adapted. The lesson does not generalise. Try the same treatment on a novel that runs on interior voice and the result is a film of people standing in rooms.
Where fidelity turns into padding
Fidelity turns into padding when a production treats a short book as though length were the virtue. J. R. R. Tolkien's The Hobbit of 1937 is a single, brisk children's novel, and Peter Jackson's version of it runs across 3 films released in 2012, 2013 and 2014, with material drawn from appendices and with characters invented for the screen, including the elf Tauriel. The films are not unfaithful in the ordinary sense: almost every event in the book is present. They are unfaithful to its pace, which was the thing readers actually loved.
The reverse case is more instructive. William Wyler's Wuthering Heights of 1939 stops roughly halfway through Emily Brontë's 1847 novel and omits the entire second generation, and it remains the most admired screen version of the book. It cut more than half the text and kept the temperature.
Why casting complaints are usually about reading, not acting
Casting complaints are usually about reading rather than acting, which is why "disappointing casting" is the most common and least reliable adaptation grievance. A reader assembles a face over 300 pages out of fragments the author supplied and a great deal the author did not. The film then fixes that face permanently, for everyone, and the private version is overwritten. The complaint that follows is real but misdirected: it is grief for a private image, arriving as a judgement about a performer. The useful version of the question is narrower. Does this performance keep what the character was for in the book, the pressure they apply to the story? A physically unlike actor who keeps that function is a better casting decision than a lookalike who does not.
The measure applied to one case
Applied to one case, the measure shows why a single verdict is useless. The 9th Life of Louis Drax, directed by Alexandre Aja in 2016 from Liz Jensen's 2004 novel, travels a different distance on each of the 4 points.
| Point | Distance travelled |
|---|---|
| Plot | Short. The case, the fall, the coma and the investigation survive. |
| Character | Moderate. The doctor's role expands to carry the enquiry the reader once did. |
| Voice | Long. The novel's alternating first-person account is replaced by images. |
| Theme | Moderate. What survives is the question of what adults do to a child and then explain away. |
Ask whether that film is faithful and the answer is both yes and no, which teaches nothing. Ask what it kept and the answer is a description of a real decision: it kept the case and gave up the boy's voice, and every complaint and defence of the film follows from that one trade.