I used to ask my students in English 102 (Written English: Forms of Literature) the following question--From the original material, to what must an adaptation be faithful? Several of my colleagues said, "Thank God you're not asking me that question."
It's a tough one.
The answer isn't bound up with casting or changes in content. Humphrey Bogart hardly looked like Dashiell Hammett's Sam Spade, who was described as a blond Satan. And Dickie Greenleaf was a painter in the novel, but a jazz saxophonist in the movie THE TALENTED MR. RIPLEY. In both cases, no big deal.
An adaptation has to be faithful to the narrative heart or theme of the original material. It also has to be faithful to the human nature of the characters.
Faithfulness to heart of the material can lead to some quite wild changes in content. Look at Conrad's HEART OF DARKNESS. Francis Ford Coppola's adaptation APOCALYPSE NOW keeps the madness and insanity of colonialism of the original, yet changes from deepest colonial Africa at the turn of the 20th century to 1970s Vietnam in the midst of the Communist/nationalist/anti-colonial war. It didn't harm the core of the story. It's still a quest. It's still a voyage of discovery. Kurtz of the story is Kurtz in the movie--megalomania and delusions of Godhood.
My father always thought Coppola's film to be the best film on the Vietnam War; so did many other Vietnam vets.
Equally good in its brazen modern appearance, complete with cell phones, texting, hacking, and forensics, is SHERLOCK, the new Sherlock Holmes series from the BBC. It's brilliant. For all the ultramodern trappings of current London, Benedict Cumberhatch's Sherlock Holmes is as frenetic, obsessive (Can you say OCD?), arrogant, cavalier, rational, and impatient as Arthur Conan-Doyle made him in the late 19th century. "A Study In Pink" is so faithful, at its heart, to the first Holmes novel, A STUDY IN SCARLET, that it sucks one in immediately. These are mysteries based in puzzles and solved logically from the evidence. How much more Sherlock can you get? And Dr. Watson, home from Britain's current adventure in Afghanistan (it looks so much like the last three) with PTSD, makes perfect sense.
Possibly the best adaptation of all time was MY FAIR LADY--largely because Alan Jay Lerner understood better than George Bernard Shaw what was the real narrative heart of the story. How do you create a human being? In the myth, the gods do it.
Shaw, who was a Fabian Socialist, took the myth of Galatea and turned into a class and gender diatribe. He called it PYGMALION, after the sculptor who created the statue with whom he fell in love and named Galatea. Shaw avoided the love story. Instead, his play is about class-stratified Britain, and the true climax of the story--Eliza's successful performance at the ball--is completely blown off. Furthermore, Henry Higgins never realizes the nature of his feelings for Eliza.
In the movie version with Leslie Howard, Shaw corrected this flaw in that it became a love story.
MY FAIR LADY ditched most, but not all, of the class commentary. Working-class Alfie Doolittle is lifted almost verbatim from the play, and his commentary is funny because it's true. Mrs. Pierce, the housekeeper, however, is almost eliminated, right along with her assertions of middle class morality. Lerner and Lowe knew they were dealing in romantic comedy, not polemic. They played with stereotypes of British bachelorhood--Higgins and Pickering--and yet made it believable romance. Eliza has her big moment at the ball where she's taken for a princess. The lady has been created. However, Higgins comes to realize he's fallen in love with his creation--"Hymn to Him" and "I've Grown Accustomed to Her Face".
While Eliza is civilized, Higgins is made human. Only love can do that.
There are two ways for adaptations to be really bad. The first way is to be slavishly devoted to the details. The second is to either ignore/destroy the narrative core of the story or to ignore/destroy the human nature of a character. While the first suffocates and bores (certain period pieces), the second is a total violation of the author or creator's work. It can be an intellectual rape.
The worst offender, in my mind, in this second category is Disney's THE JUNGLE BOOKS. Even while I concede it's a kiddie flick, I hate what they did to Kaa. Disney reduced Kaa to a Western stereotype--snakes are bad. In this case, Kaa wasn't evil, he was just silly with an inane voice. Even as reference to the biblical Serpent, it was an insult.
Kipling's Kaa is a very different creature. A character borne of Indian sensibilities, Kaa is the rock python in whose coils Mowgli sits to learn. Kaa is the embodiment of wisdom, and wisdom is a three-edged sword. In the battle with the monkeys, the reader can see that wisdom has its terrible aspects. Wisdom is like all things: it is the use to which it is put that determines whether it is good or bad.
Another example is the latest STAR TREK movie. It was fun. It wasn't STAR TREK. It violated almost everything we know about Vulcans, Romulans, and StarFleet.
I think adaptation is a perfectly valid artistic approach, but I do come to them cautiously.