A frequent theme in wartime propaganda posters is the idea of civilians limiting careless talk, as there could be spies listening. Would the average person have access to any information that would be useful to a spy? Is there any truth to the idea that any casual conversation could be overheard?

by Shark_Girl_Jazzy

I'm specifically wondering about world-War Era times, but other time periods are welcome.

Flammy

An answer to a similar question from a month ago includes some links to additional prior answers until we have a more well rounded answer.