There are many types and styles of scripts, the one you choose depending on what you are writing a script for.
Just like Treatments, scripts are always written in Courier font size 12.
Screenplay
A screenplay is a common style for video projects that have a narrative and includes dialogue.
suggested-script-layout.pdf41.6KB
The Dark Knight Opening Scene.pdf153.7KB
Online Script Writing Software
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Example Scripts
Scriptwriting & Pre-production Tools for Video and Game Production | Celtx
Online Video, TV & Film Production Management Software and Screenwriting. Manage cast and crew, create and share call sheets, script breakdown sheets...
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