One thing that a film always does that is different from what a book can do--which may be either a good or bad difference--is to render the narrative to an entirely visual experience (turn off the sound and you still have a meaningful experience of a visual narrative). The camera intrudes--or enters gracefully in--as a character originally absent from the narrative: the camera sees and interacts with the narrative giving it the dynamic of a character. Even a passive camera that is fixed in one place, does not move in any direction, doesn't zoom in tightly or out loosely, etc. gives a unique visual representation to the narrative the book cannot give. The significance of this is that the imagery in the film--and the memorability of the imagery--will be radically different from that in the book. The significance of this is that the viewers perception and understanding of and interaction with the narrative will be radically different. Thus it is that authors can, and do, say along with Alice Walker that the "movie is not the book."
One of the difficulties in trying to portray a novel like this as a film is that the narration of the novel precludes an accurate reflection. Note the way that the novel is narrated from the point of view of Celie, and although the film tries to replicate this by having a voice over, narrating her thoughts, the book gives us a far deeper insight into the character of Celie than the film does.
While the movie tries to stay true to Walker's novel and overall message, even Alice Walker herself is quick to point out that "the movie is not the book." Partly because the time in which the movie was produced, but the relationship between Celie and Shug is downplayed in the movie version of The Color Purple. Spielberg's version does have the two women develop and strong and beautiful friendship, but it is not taken to the deep level that it is in the book.
The structure of the movie is also different than the structure of the novel. The book is composed of letters Celie writes to God, but the movie doesn't show this. The book shows Celie's journey into womanhood as she talks through her thoughts, feelings, and emotions with God. The movie leaves this part out, mainly because it would have been difficult to take the time in the movie to set this up.
