The verb "to adapt" means "to transpose from one medium to another," while adaptation is defined as the ability "to make fit or suitable by changing, or adjusting," according to Syd Field in his book "Screenplay." This changing and adjusting process entails several alterations from the source material to the end result, including changes in structure, function and form.
Adaptations must be considered as an original work based on other material, according to Field in his book. Those adaptations that are too literal and which closely resemble the original material seldom work, he adds. Although exceptions exist, in many cases when a movie screenplay adheres too closely to a source novel, scenes that may play spectacularly through the written word don't translate as dramatically when conveyed through moving pictures and sound.
Because different mediums have different rules on what may work or not during a given narrative, changes must frequently be made from the source material. In many instances, whole characters, scenes and even substantial segments of the source material are excised, while others are added in. However, the integrity of the story and the themes are left relatively intact in the process.