The most obvious difference between a novel and a movie script is the visual format. A movie script follows strict guidelines in its appearance.
Then a movie script is always written in present tense, “he goes”. Most novels are written in past tense, “he went”.
The most difficult thing to grasp is that a movie script only describes what can be seen or heard. There are no thoughts and no backstory.
This may sound simple, and technically it is, but you also want to transfer information which you in a literary work can present like: “The young woman was Sarah and she was half-sister to the princess and dreamed of world peace while she worked as a shepherd. Her best friend was Tommy who she hoped to go to the moon with one day.”
In a movie script all you would be able to write of all this is a young woman herding sheep, because this is the only thing visual. (The name is written in a movie script even if it is never told, just because of readability.) All the other things have to be told in dialog or shown as the story passes by. And it is far better to tell information by adding visual things, than have it spoken.
How to show that she is half-sister to a princess? Well, that depends of course, but one way is to have a fancy car stopping by, the driver opens the door for a young woman in tailored clothes. Our shepherd waves and calls “sis”, but the other one is less eager to return the enthusiasm. When they talk she may say something like “mother sent me to tell you XXXXXX, but father disapprove.” Our shepherd replies “I don’t care what your father thinks.” Yes, this is spoken, but it is not “I’m only your half-sister and my father is the king, yours is a farmer” There are ways to pass information as a combination of visual things and spoken.
The basic idea is it that the movie script should be easy to read, so it is possible to write that Tim is Jacob’s son, especially if they are in the same room and presented in the same time. After all the script is there to help the actors and there is no need for confusion. It is far better to write such things and leave it to the actors to express these things, than write every gesture in the script. One could say that these are visual things compressed into “Tim is Jacob’s son”.
One other thing that a movie script differs from a novel is the time frame. In a movie script one page should equal one minute of film. This means that if you write a lengthy description of a room, you actually guide the camera around the room showing the collection of butterflies, the mahogany bookshelves, the dust of the fruits in the bowl, the pattern on the Persian carpet and so on. This takes time. And transferred to a movie it may become very uninteresting and tiresome. So only keep what is vital for the story and frame the essence of a situation in as few words as possible.