Let's talk about setting. The tale that's been rolling around in my head takes place in a bed and breakfast type deal on an island. They're cut off from the mainland. The last boat out has left. There will probably be a storm. (Hint: there will definitely be a storm.) The point is, it's a
Closed Circle. Nobody comes in, and nobody gets out. That keeps characters from doing pesky little things like going for help.
"That is such an overused trope!" you might say.
And you might be right. If you look at the examples on the page I linked, there are a ton. Maybe that's because it's super convenient when you don't want your characters wandering off. Maybe it's because it's a lot of fun. You can decide for yourself. For me, it's a bit of both.
Because, see, if you have a way out, insisting you can't leave gives you a little thing called a false dilemma. Which is something I find irritating and try to avoid. Say my options were "try to find the killer and stop them" or "wait around until they kill us too" while there is a perfectly good way off the island. That's not going to work for me. In my case, the choice is more like "stay here" or "swim a few miles in the frigid Pacific."