It is not uncommon to picture the characters and setting while reading the book; this is an imaginative process that is unique to every reader and makes the story special to them. The creation of a movie might destroy their own mental “movie” of the book as there are bound to be discrepancies with one’s imagination. As a result, the experience of building up one’s own interpretation of the story from reading the book is lost and limits one’s imagination. The fun in reading a book for the first time disappears.
Some people may picture the story better, or even feel it would be more realistic in their minds, if they have seen a predetermined visualisation of the world through a movie. Taking advantage of the fact that movies may not faithfully recreate every element from the book, readers can now build a new mental "movie" that shows the original story, in its complete authenticity that the author wanted it to be.
[P1] People imagine the story, constructing the world in their minds as they read the book. [P2] A movie takes this imaginative aspect of the process of reading a book away.
Rejecting the premises
[Rejecting P2] A movie does not necessarily ruin the imagination in reading a book.