Names must be given to the major characters and the important minor characters. Characters that appear only shortly may be given names, too. Not giving a name to a character signals the reader to not focus on them. If the function of a character is more important than their individual identity that function can be used to name them: TECHNICIAN, COP 1, COP 2, PROFESSOR etc.
If a character changes his name in the course of the script or if there is a case of mistaken identity the most important issue is to not confuse the reader ( audience = reader ). One way to handle this is to use both names after the change, like in JAKE/BILL or JAKE (BILL).
However, if it is clear to the reader it might be a good idea to use the true name of the character throughout even if he acts under a different name. The Screenwriter’s Bible quotes the script of ‘North by Northwest’ to make that point.
These are the character names that precede the dialog lines, like JACK and MIKE in the examples above.
They are written with all capital letters of size 12 Courier or Courier New fonts.
As specified on the page on screenplay measurements, character cues must start 3.7 inches from the left side of the paper that is 2.7 inches to the right of the left margin (which 1.0 inch from the left side of the paper).