Sometimes when I'm reading a script I'll get to around page 70 and come across a sentence like, “Tom arrives at the shop and goes inside.” There’s nothing inherently wrong with that sentence, unless the writer only introduced Tom briefly on page five and hasn’t said a word about him since. I’ll know from the fact that Tom’s name isn’t in all caps that he’s been in the script before, but there’s no way that I’ll remember who he is if the writer hasn’t mentioned him in 65 pages. If this happens and the script is in PDF format, then I'll search for the name “Tom,” and try to figure out what’s going on, but if it’s a paper script then I’m definitely not going to bother.
Imagine if you met someone in kindergarten for a few minutes and then he just turned up at your door 65 years later and said, “Hey, what have you been up to?” Other than the many glaring differences between these two situations, they're exactly the same! If you ever do this kind of thing in one of your scripts, be sure to give the reader a hint about who that particular character is if you haven’t mentioned him in a long time, such as by saying, “Tom, the musician who shot Glen, arrives at the shop and goes inside.” Your reader will thank you.