I am writing a screenplay for a TV show in which a character who will become important later in the series has a line of dialogue in the opening scene. However, he is offscreen and will not be physically introduced for another episode or two. For the dialogue, is it okay if I have him named as “VOICE” or “MAN” even though he is a main character? Or should I use his name for the dialogue, but not give a description in a slug line until he is visually introduced? It is inferred in the scene that the person he is talking to has no idea who he is, and for the benefit of the script reader, I would prefer to keep the mystery intact. Thank you!
Screenplays are production documents.
Screenplays communicate things that are intended for production like who says what line, so casting agents, directors, and actors know who is present in the scene without second-guessing the author or discovering it episodes later. The producer/director/casting agent of the first episode may never read any of the later scripts.
If you want this to be a specific voice from a specific character played by a specific actor, the screenplay needs to be clear it's his line.
When you introduce a character who later has a name, use the character name from the start and always call them by their character name. If you have a speaking role that has no name they can be MAN IN OFFICE or WOMAN IN RESTAURANT – but keep it consistent throughout all scripts.
If you want the reader to make the connection between the unannounced character's remark and his further introduction, you could probably define the character with a bit more certainty than just a MAN or VOICE, such as describing the MAN as tall, strong, sinister, of the VOICE as loud, hissing, crackling, or baritone, for example. Perhaps, use something that the character will be later known for. Other than that, it is totally up to you.