I'm not sure what you mean by a difference in writing a script and screenwriting? Are you asking about the difference between a stage play and a screen play?

Always use Courier 12-point font.

Left Margin is 1.5 inches (appx.15 spaces)

From Left Margin justified left tabs:
Character name is appx. 25 spaces
Parenthetical is appx. 21 spaces
Dialogue is appx. 15 spaces

Dialogue should not run longer than 3.5 inches.

Use an unjustified 1.0 right margin.

1.0 inch top and bottom

There are five main elements of a properly formatted script

Scene Heading/Slugline
Action
Character Name
Dialogue
Parenthetical

Sluglines or Scene Heading
Written in capitals and containing three pieces of information: (1) Where; (2) Exactly where, and (3) when. (2) and (3) are separated by a space, a dash, followed by another space.

(1) can be INT. (interior) or EXT. (exterior); (2) is a short identification of the place; and (3) can be either DAY or NIGHT.

For example:

INT. AIRPORT - DAY

You can use more than one subject.

For example:

EXT. AIRPORT - TICKET COUNTER - DAY
EXT. AIRPORT/TICKET COUNTER - DAY

You need a new slugline each time you change the place, and/or change the time.

Action, or Description
The screenplay?s visual elements, where we show what is taking place on the screen. Character descriptions, what they are doing, the places, and everything the audience will need to assimilate visually. Don?t use capitals for sounds, props, visuals or emphasis. But you will capitalize the name of each character the first time you introduce them.

Character Name
In a properly formatted script the Character name is placed above the dialogue, in caps appx. 25 spaces from the left margin; not centered

Dialogue
This is where you place everything the character says.

Parenthetical
Do not use parantheticals under the character name to show action. And don?t use them to tell the actor how to read the line. Let the emotion come from your script and dialogue. Try to avoid using them at all. But if you must use them they go under the characer name appx. 30 spaces from the left margin.

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The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

wow, thanx...that helps a lot. i just wasn't sure if there was a difference between script writing and screen writing. but you sure did take care of the script part. ?:D?