This is something that cuts me to my core; when someone is meant to be cut off mid-sentence but they aren’t. It just kills me a little every time. It happens all the time and it’s painful to watch. It seems to usually happen because either nobody picked up on it being written in or the lines weren’t well enough memorized to compensate for it.
As a general rule I like to plan out an approximate time to cut someone off so that they can end their line at the right time. If I’m the one being cut off then it’s a different story entirely; if you’re supposed to get cut off, and don’t, then you have been given no reason to stop speaking (unless you justify it with something else happening that would stop you) and the whole thing looks like a mess if you just stop for no reason.
The simplest thing I’ve thought of to prepare for this (because it will happen to all of us at some point) is to be ready to keep talking until your scene partner remembers that they’re supposed to jump in on you. Of course, if the director (or possibly playwright if they’re involved) says not to do it, then you just have to trust your scene partner to get it right.
Some writers do that for you, for which I’m grateful. The others will generally use a hyphen to indicate it, though I’ve seen it done with ellipses or just about anything you can think of that might indicate that people are either speaking at the same time or cutting one another off.
I guarantee you that at some point in your life you will have a scene with somebody who just forgets that you don’t finish your sentence. There will be a long, uncomfortable silence. If you’re lucky it’ll look like it’s done on purpose. If you’re not, then at least it’ll probably never happen again with that person because they’ll probably be embarrassed. Don’t be too hard on them, because I can also guarantee you that someday you’ll be that person, and it sucks. It really sucks.