Write the author's last name at the beginning of the citation line. Place a comma after the last name, then add the author's first name. Put a period after the name is completed.
Write the name of the film you are quoting in italics (after the name of the author) followed by a period.
Write the publisher's name, if there is any. In most cases, the script of the movie will not be a published document; hence, it should be treated like an unpublished manuscript. In such a case, after you write the title of the script, you need to add the words, 'not published' or 'unpublished'. Follow that with a period.
Write the year the film script was created after the script's publisher. If this is not available, write 'no date'. Mention the page/ pages you are quoting at the end in the following format -- for example, 'pp145-147'. It should look like this: "Ferguson, Allan. 'The Alien'. Unpublished manuscript, 2002. Pp 145-147."
Use steps 1-4 to cite information/quotes obtained from a film script that is available online. The only difference here would be that after the year of completion of the manuscript, you would need to add the URL from where you accessed the said script.
The citation would read: "Ferguson, Allan. 'The Alien'. Unpublished manuscript, 2002. http://www.TheAlien.com/Script.html" (the website quoted here is fictitious).