If you want to write a screenplay, you'll usually need a story to tell and an understanding of the method of storytelling used in cinema, which is a visual medium. It can be helpful to read any scripts you can get your hands on, and some people greatly benefit from buying books on screenwriting. Many individuals also watch a lot of movies and pay attention to the way film stories are structured because they are often paced much differently than novels or short stories. Another key aspect for learning to write a screenplay is to understand the format used in screenwriting. This will probably be covered pretty heavily in any books you read, and it can vary a bit between different screenwriters, but there is an industry-standard way of presenting a screenplay, and knowing how it works can be helpful.
Since film is a visual medium, it can be difficult for the filmmaker to get into the thoughts of his characters, at least in a literal way, without using narration or imagery. As a result, screenplays tend to focus on descriptions of character behavior and dialogue. These things are often much more central to the storytelling in a movie than they are in a novel. In fact, the average screenplay is broken up into only three kinds of writing: scene descriptions, action descriptions, and dialogue.
The scene descriptions are usually presented after a scene heading, which is also known as a slug line. A typical slug line might look like this: "INT. Office – Day." The "INT" in this case, stands for "interior;" it could have been "EXT" for "exterior." These distinctions are mostly helpful for allowing the future filmmaker to break down scenes into those which will be shot indoors and those which will be shot outdoors, a consideration that can be important in movie production. Beneath the slug line, the screenwriter will lay out what the scene looks like; it is usually written from a third-person present tense perspective, instead of past tense like most novels.
The second part of trying to write a screenplay is to describe the action. This is also usually handled in the third-person present tense. An example of an action description might sound a little like this: "Carrie opens the drawer on her desk and removes the document, handing it across to Bill." Sometimes there will be some subtle — or specific — camera directions mixed into the action as part of the storytelling, but screenwriters who expect someone else to direct the material often avoid these to make the screenplay less distracting and easier to read.
The thing that takes up the biggest part of most screenplays is dialogue. Sometimes there are several pages of dialogue between each action or scene description, although this does depend on the kind of film being written. Dialogue is generally presented with the character's name centered on one line, and the actual lines for the character written below.
Usually, dialogue is about half as wide on the page as the action descriptions. For example, the margins may be set up so that the action and scene descriptions are 6 inches (15.24 cm) wide on the page, and the dialogue might be set up to be about 3.5 inches (8.89 cm). Sometimes there are little descriptions of the behaviors a character should be exhibiting while speaking in parenthesis on a separate line between the name and the dialogue.
Another important issue to consider when trying to write a screenplay is the question of pacing. Films are generally paced much more quickly than other forms of fiction since they are designed to tell a story in one sitting. Screenplays generally move more quickly than novels, for example, and time-compression techniques, such as montages, are used to quickly summarize things that might be dealt with in detail if someone were writing another kind of fiction.