I have a character that appears midway through the script, but is never introduced by name and the reader should not know who he is at this point. So, let’s call him something descriptive: ONE-LEGGED MAN. All the while, in other scenes, his actual name is being mentioned. Let’s say: KEVIN SUGARMAN. Towards the end of the script he introduces himself to a character and it becomes important that the reader understands that ONE-LEGGED MAN is KEVIN SUGARMAN.
From this point out what do you think would make for the smoothest read:
1. Continue calling him ONE-LEGGED MAN
2. Call him ONE-LEGGED MAN/KEVIN SUGARMAN
3. Or start calling him KEVIN SUGARMAN
This happens in scripts all the time. There’s no perfect solution, but your general goal should be to confuse the reader as little as possible for the fewest number of pages.
If One-Legged Man has dialogue as “ONE-LEGGED MAN,” keep using that name through to the end. It’s confusing to have dialogue blocks with differing names.
If One-Legged Man has no dialogue (or very little dialogue) before he becomes Kevin Sugarman, it may be worth swapping his name, particularly if he hasn’t been prominently featured in a lot of other scenes. The slash technique (One-Legged Man/Kevin Sugarman) works best in scene description, and then only as a reminder to the reader. The guy’s name shouldn’t be 25 characters long every time you use it.
Finally, there are times when the best solution is to simply tell the reader that the character’s name is Kevin Sugarman from the time he’s first introduced. From what you’ve described, it sounds like the reveal is very important to your story — it a key joke or plot point. But in many cases, it may not be worth the trouble and possible confusion.