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The way to foreshadow something without giving it away is to use it to tell something else. If you tell there is a gun, people think it might be used to shoot someone. If you tell that a person is ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/43492 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The way to foreshadow something without giving it away is to use it to tell something else. If you tell there is a gun, people think it might be used to shoot someone. If you tell that a person is a police officer who takes his gun off when he comes home? Well, somebody is still probably going to get shot and you are foreshadowing it. But what if the reason you mention him taking the gun off is because years ago he accidentally shot an innocent man in the line of duty and every single time he takes the gun off he remembers that. And you can see that going thru his mind. In your particular case since they communicate by letters, the easiest way is to link it to something communicated in those letters. Not that you need to specifically describe or even mention the letter. So there is some reason that door is locked. And the other person does not want the MC to know that reason. And he does not want the MC to think about it and try to find out. So he will provide a perfectly reasonable reason why he needs a locked basement and why it would be a problem if the MC went in. The supplied reason would almost have to be related to work, so that it is serious enough to justify the locks and the MC thinks the other person is stuck with it and cannot make exceptions. And it is perfectly normal and totally non-suspicious to mention what people do for a living when you first mention it. And it would be natural to mention what silly things are involved with that work. I mean, a perfectly secure work room in the basement you cannot let anyone else in? This guy sure is committed to his work. Some hobbies might also justify the locks. Say if you have a valuable collection of some sorts in there. This is not always a good choice because people will expect you to show off the collection when you have the chance. But what if there never was such a chance before things start happening and the other character knew that in advance?