My grandmother told me that growing up, you had to be very careful what types of topics you talked about out loud, even with friends or colleagues from school. “The KGB was everywhere, you never knew who was an informant.”
As proof she told me about one night when she was walking to a subway stop with friends, some close some not, from medical school. One of them, (a fellow jew, she was always sure to add) made a joke about Stalin. He didn’t show up to classes the next day. Or any other day, for that matter. They never saw or heard from him again. He just vanished. This would have been in the late 1940s.