What is the difference between a literary text (such as a short story or a novel) and a film?

A short story or novel can be read in as much or as little time as the reader wants to linger over the details. The reader fills in the gaps of the writer's descriptions. A film, by contrast, has a fixed length (We can disregard the opportunities new media give us to play back favorite scenes or fast-forward through others) and the viewer is spoon-fed the visuals. One isn't necessarily better or deeper than the other, but they are not comparable experiences.

Consider Frankenstein. It has been adapted to film on many occasions and some of these films are wonderful art in their own right, but none of them accurately adapts the novel. The 1931 film (probably the best-known) keeps a brisk pace, is set in an era contemporary to that of the viewers, shows a monster with a flat head and bolts in his neck, and gives us a shorthand account of the relationship between Victor and Elizabeth. In the source novel, set in the 1790s, the reader is brought along as the story is told at a leisurely pace. We're given vague and inconsistent physical descriptions of the monster. Victor spends long stretches of time taking "coaching tours" of Europe while considering the practical issues of making a monster, and another one to consider how to craft its bride. From his lab in Ingoldstadt and from his coaching tours, he writes passionate letters to Elizabeth but actually only spends a few days total with her during the course of his adult life. This is ideal for the various ideas and themes Mary Shelley develops, but these elements make for terrible cinema. The action scenes are far apart and only last a few pages each. Frankenstein is difficult to faithfully adapt and it might not be worth the effort to do so. Film and prose are different media with different needs.

Again, one isn't necessarily better than the other. Sometimes the book's author also writes the screenplay for the film, and both are of comparable quality (True Confessions and The Pope of Greenwich Village are examples of this). Sometimes the film version is actually superior to the book (The Godfather, for example, or The Graduate). This usually happens when a strong film director adapts a weak novel or story.

Ultimately, a prose work has a single writer as its sole author. A film is a collaborative effort involving dozens or hundreds of participants. Its primary author is the director, not the screenwriter, and it will involve a lot of compromises from the director's original vision (and even more from the script, or the source story if there is one). A book is generally the purer vision of a single writer. This alone is a huge difference between the two media.