I've only just read Dodie Smith's novel 101 Dalmatians. As I'm breaking the film down, I was curious to see how it compared to the source material. While I am a long-time admirer of Bill Peet, his adaptation of 101 Dalmatians is an excellent measure of his talents and only increased my admiration for him.
The book has many differences from the film that resulted. The dogs' owners are the Dearlys, not the Radcliffs, and they never get first names. Mr. Dearly works for the government in finance; he is not a musician in any way. There are two nannys, one who worked for each of the Dearlys before their marriage. There are three adult dalmatians: Pongo, his bride Missis, and a wet nurse named Perdita. Cruella is married to a furrier and her plan isn't just to own a dalmatian coat but to go into the manufacture of them. Cruella has a Persian cat whose kittens she keeps drowning. The Badduns are brothers: Saul (not Horace) and Jasper.
The book begins with the Dearlys already married. When the puppies are stolen, it happens off-stage. Cruella is visiting and waiting for the Dearlys to return. While Cruella keeps Nanny busy, the dogs are stolen without any confrontation whatsoever. Tib (not Tibbs) is female and her real name is Pussy Willow. The horse in the book is not associated with the Colonel and Tib but is part of a Gypsy encampment. The end of the book does not include a chase sequence. Once the dogs board the van heading to London, they make it to Regents Park without further incident.
The book spends an awful lot of time on logistics, dealing with how the dogs are going to find sufficient food, water and shelter. The humans in the book, including Cruella, are developed in general terms only. None of them is particularly vivid as a personality.
I mention all of the above to show how much work Peet had to do in adapting the book. He was forced to invent a lot and restructure a lot of what was left. He streamlined many of the plot points and incidents. One of the best things he did was to eliminate characters, cutting out Missis, one nanny and Cruella's husband. He understood that he needed scenes that included conflict and suspense. The opening of the film is a lovely sequence that Peet created out of whole cloth. The kidnapping of the pups includes a direct confrontation between Nanny and the Badduns and is far more interesting than what's in the book. The final chase is also far more exciting than the novel, where the climactic tension comes from trying to move so many puppies over a great distance while keeping them fed, watered and rested. Besides increasing the threat to the dogs, Peet includes the point of view of the villains where the novel only sticks with the dogs.
It's in the area of personality that Peet really shines. Cruella is distinctive in the book, but she lacks the flamboyance that Peet gives her. Roger is far more interesting as a musician, which instantly gives him physical business to do, than Mr. Dearly is as a financial advisor in the book. The Badduns are ciphers compared to Horace and Jasper. The relationship between Tibbs and the Colonel is better developed in the film and Tibbs is given a greater role to play.
Peet took the novel only as raw material. He kept the central conflict of the book and what worked cinematically, like the twilight bark and the pups' interest in television, but pulled the whole thing apart and rebuilt it adding drama, suspense and personality. Anyone having to adapt a story for animation would benefit from comparing the novel of 101 Dalmatians to the resulting film. While the Disney film is admired for many things like the art direction and the animation, the underlying appeal of the film really has to be credited to Bill Peet. He's the one who gave the film its overall shape and appeal.