Scripting a story can be an organic process, and methods of screenwriting will vary depending on the writer. You’ll see the variances between completed scripts of different scribes. A shooting script, however, is not a completed work, like a novel is.
A shooting script is a road map – a set of instructions to be really basic about it – to a movie yet to come. That’s why clear and consistent formatting of the shooting script is essential.
What if the character is “unknown” when first appearing?
When, for example, the character “Fred” is first off screen and speaks the lines labelled in the script as “Voice” or “Unknown Voice”, revealling himself later in the scene to the rest of the on-screen characters… well, the set crew cannot be certain the “Voice” lines are Fred’s and not someone else. To retain the writer’s intention, and to make the road map clear to the shooting crew, in this type of case, label Fred’s off-screen lines as: “Unknown Voice (Fred)”.
What if the character changes names during the story?
Say “Fred” began in the story as “Mr Smith” and only became “Fred” as the other characters got to know him. As your guide, consider… will the different characters (2 different character names) be played by two different performers or by one? If the answer is one performer, then use the same principle as for the “Unknown Voice” above. In this example, start “Fred” as “Mr Smith (Fred)” and let him become “Fred” as the characters get to know him. Alternatively, of course you can label him as “Fred Smith” all the way through the script; however, with this choice, you’ll never really know when the writer intends for Fred to change his name among the other characters in the story.
Overall, remember that yes, you’re creating a road map (of instructions) for the shooting crew, but you must try to keep the writer’s intention wherever possible. The writer’s intention is an integral part of the road map too!
Cheers & happy script formatting to you!