When introducing a character in a short film script, does a full name have to be used, or is their last name not needed?
Typically, scripts will, on first appearence of the character, introduce them by their full name, and from then on, they will use a shortened form of the name. If the character is "John Smith" he will be given his first line of dialog as "JOHN SMITH" and from then on be "JOHN". If two characters have a similar name (John Smith and John Doe) it may be prudent to use "SMITH" and "DOE." Or if one is a regular and one is a guest, keep the regular's typical name, but use the last name for the guest. Names should also be consistent with character's prefered name (I'm sure in X-Files, Mulder was always scripted as MULDER and never FOX because the character hated his embarrassing first name... only his parents and when his formal legal name was needed only referred to him as Fox.)
If your character has a duel identity that is going to remain hidden from the cast to prevent leaks, it might be wise to use the false name until the character's true nature is revealed OR print a second copy of the script for the actor only that uses the real identity.