There are many differences between The Color Purple the novel and The Color Purple the film.
We might discuss how the film mostly reduces the men to either good or bad, while Alice Walker makes the men much more complicated.
One thing that struck me right away was how tall Mister (Mr. ___) was in the movie. Why? In the book, Celie describes Mister as "small like his daddy."
This might seem like a tiny difference, but think about the stereotypical bad, abusive man. Do we think of him as tall and commanding or small and diminutive? Can't a small man be just as terrifying as a large man? Also, can't an abusive man—a man who's done lots of wrong—change?
Walker's novel allows for such dynamism, but Steven Spielberg, the director of the film adaptation, precludes such intricacy.
While we're on the topic of intricacy and complexity, let's talk about Celie's sexuality. Is Celie a 100-percent straight woman? In the novel, she's not. She has an affair with the singer Shug. What about Shug? Is Shug a 100-percent lesbian? It'd be hard to argue that she is, given that she married a man and falls in love with another young man.
Anyway, this is all to say that in Walker's novel, Celie has a fluid sexuality, while in the film it's much more stagnant and static.
It might be interesting to talk about the film's choice to all but clip Celie and Shug's romantic relationship within the context of bisexual erasure. It might also be worth wondering: If Celie and Shug were white women, would Spielberg have been more comfortable clearly showing their affair in his film?
In addition to examples given in previous answers, I would say that a major difference between the book and the movie directed by Steven Spielberg is that the movie essentially cuts out the entire section of the book where Celie and Shug live in Tennessee together. We see Celie finally leave Mr.—, we see her attend her step-father's funeral and inherit his house, and we see her later on after she's started her successful business, but we do not see what happens in between.
In Alice Walker's book, we get to see Celie as she lives in Tennessee and develops her company, Folkspants, Unlimited. This business gives her financial independence as well as a way to express herself.
The movie also completely cuts out the emotional plot point (in Letter 83) where Shug breaks Celie's heart by telling Celie that she also wants to have a relationship with a young man called Germaine. Because of this, Celie decides that she can no longer live with Shug in Nashville, and returns to Georgia. Getting over this pain helps Celie to move on from her dependency on Shug, as well as find common ground with Mr.—.
One other significant difference between the book and the movie is that, in the movie, Shug Avery is made somewhat more vulnerable than she is in the book. In the book, she comes to the home Celie shares with Mr. _____, sick and in need of nursing; it's the same in the film. However, the movie creates a conflict between Shug and her father, a minister who disapproves of her lifestyle and career choice as a performer and singer. In the movie, Shug seeks his approval, something which he continues to deny her. And she moves on, but it seems to be a soft spot for her forever. In the book, there is no such conflict or vulnerable soft spot.
Further, the pants that Celie learns to make are a major symbol in the book, and she makes pants with all kinds of fabrics for everyone: both men and women. The symbolism seems to be somewhat lost in the film. Much less focus is placed on Celie's business in the movie than it is in the book.
- Alice Walker's novel The Color Purple is much darker and more complex than Speilberg's adaptation, which develops the comedy and the musical aspects more. In the book, the men are meaner, crueler, more abusive, more sexist. In the movie, they seem more like childish clowns, especially Mr. ____. The book reads like tragedy and ends up like comedy only at the end. The film introduces the comic/musical aspects much earlier and develops them throughout.
- Celie is a writer in the novel: she writes to God. It's an epistolary novel. Letters to God is the impetus of how the novel beings, as confession. There's no such writing or confession in the movie: it's interior monologue done as voice over. Speilberg only shows her writing to Nettie, not to God.
Some other, smaller differences:
- There's no quilting in the movie. It's a major motif in the book.
- The movie shows more female's kissing (homosexuality) than the book (there's only Shug and Celie), which takes away from the power of the sexual experimentation.
- Again, the music is a major character in the movie (Quincy Jones was brought in to do the score), and there's more hymn singing and juke-joint cross-cutting to make it seem more like a musical than a novel. Obviously, this is a major reason why it becomes a musical on Broadway later.