Screenwriter

A screenwriter's role is to write the screenplay for the film. Th8s means that the screenwriter is in charge of writing what the audience can see and hear on screen. The screenplay is usually up to 90-120 pages long (1 page is equivalent to a minute of screen time), therefore a film with a page count of 120 would last for 2 hours.

Elements of a Screenplay:

Scene Heading:
Scene headings (also referred to as sluglines) are aligned to the left, about 1.5" from the edge, and are rarely long enough to reach the margin. The heading are wrote in capitals, with the use of abbreviation EXT. for exterior and INT. meaning interior.
The use of sluglines are to allow the script reader to understand where the scene is located and the time of day in which it is set, allowing the reader to imagine the scene. This line may also include details for the production team such as the type of shot which is to be used. Source

Star Wars: Episode III - Revenge of the Sith (Source)

Action:
Action runs from the margin to the right and is the full width of the page, this should be wrote in real time. The description or action sets the scene by describing the setting and allows the introduction of characters and sets the stage of the story. Source

Star Wars: Episode III - Revenge of the Sith (Source)

Character Name:
The character name is in uppercase on the left hand margin. The writer inserts the characters name to allow the reader to understand who's dialogue is to follow. A character's name maybe the a name (NATHAN) or a description (SKINNY TEEN) or the characters occupation (TEACHER). Source

Star Wars: Episode III - Revenge of the Sith (Source)

Dialogue:
Dialogue indicates when someone in the scene speaks. This could be a conversation, a character talking to themselves or a voice over. Source

Star Wars: Episode III - Revenge of the Sith (Source)

Parenthetical:
A parenthetical can be an attitude the character has to feel, a verbal direction or action direction the actor has to follow whist reading the lines. These directions should be short and to the point, and to be used when necessary.
A parenthetical may also be seen in scripts as (continuing) this is because a piece of action has taken place the characters line. Source

Star Wars: Episode III - Revenge of the Sith (Source)

Extensions:
An extension is a technical note which is placed to the right of the characters name and describes how the character's voice is going to heard. For example V.O. stands for voice over, a voice over is usually narrates the screen, or a character speaking while not in the scene. Source

The Imitation Game (Source)

Shot:
A shot tells the reader that the focal point of the scene has changed. For example, close up, pan or point of view. Source

Title Page:
The title page is in the centre of the page, the title page includes the title of the film and the writer and co-writer. In the lower right corner screenwriters include their address, and in the left a registered, WGA or copyright information. Source

The Imitation Game (Source)