Just as movies do things novels can't do well, novels do things movies can't do well. Movies are just pictures, with sound, and have a time limit. Novels could go on forever, really, but a movie rarely goes past 2 hours. When writing a screenplay it's important to keep in mind the things a novel does well that don't translate well to a purely visual medium.

Feelings

Feelings are communicated to other people using words. Movies can communicate words but as many as novel. In movies they will try to communicate feelings through moody photography or the facial expressions of a character but aren't accurate. Not to say that there isn't some value in keeping the character's feeling ambiguous, just to say movies can't make them concrete like a novel can.

Thoughts

Along with feelings are other internal self-communication that is more often than not in words. Though there are some pictures that might accompany thoughts, the structure of them is so disjointed that they can't elegantly be conveyed on screen. If you really want to know the thoughts of a character they would be a drastic change from the visual style of the rest of the movie and would still be contrived.

Information

Novels are good at succinctly communicating things movies try to mimic with it's pictures. The problem is movies are such an engrossing, all-consuming experience that even a little side information can take away from the experience instead of adding to it like in a novel. Movies are about simplicity and streamlining, novels are about giving as much information as possible. This information may be interesting and might enrich the experience but it may not be best to include it in a film.

Summaries

In prose you can quickly tell stories that would take longer and be less clear if visualized. "He walked 10 miles" isn't something you can't necessarily precisely communicate in pictures. You could always just add voice-over but this is antithetical to the big screen experience, which is a noisy environment that should be about images and not words. Where a novel can tell you someone "picked up some milk, took a swim, picked up his daughter at school" in a three seconds this might take minutes of screen time.

Descriptions Of Non-Visual

Movies do better than novels in describing visual things, but it isn't good at conveying what the other senses perceive. Movies can't really tell you how something tastes in detail though it can show you a picture of the food. A film can't really give you an accurate idea of what something smells like even though you can see the scent wafting. These things take lots of words to describe and this isn't something a movie has time to do.

A movie should be about pictures with words to jump in where picture don't deliver. So instead of writing a movie that will be struggling to be a novel, write a story that it can tell best. Take the essence of the novel and come up with a story in that world that begs for a screen to tell it.