One way to stop people from talking about something is to censor them. For example, if a heavy-handed government wants to get away with a crime, they can censor everybody who tries to talk about it. If you can silence people for long enough, enough people will forget about it that even if the information does finally come to light, nobody will do anything about it. Or, the people who committed the crimes will have been out of power for so long that doing something about it would be pointless (eg. Henry Kissenger).

A more insidious method of controlling information is self-censorship, where, in a climate of fear, people don't talk about things in an attempt to save their skin!

Self-censorship is insidious, because if I see that you're too afraid to talk about something, I will assume that if you talked about it something will happen to you, therefore I won't talk about it either because of the fear of something unknown, which you've implicitly communicated to me.

You can see this kind of behaviour in the news media in the United States, as well as among congress-people. They want to speak-out against the government, but end up censoring themselves for fear of losing their jobs. This perception was started because of the Bush propaganda that those who don't support a war against another country are unpatriotic, but that was only really applicable early after 9-11. At this point, it's just inertia and fear that stops people from speaking out.

The public at large is guilty of the same thing. If we were to vocally question policies and behaviours, people would start to notice their flaws and changes would have to be made. However, since nobody is questioning, nobody even notices these things!

Insidious.