Our mind is a wonderful instrument.. It is never still, even in sleep there are dreams. What we put in it is important. There is even a part of it called the subconscious mind we can program through emotion, or listening when we are quiet inside. If you will yourself to listen to the thoughts that run through it all day long, it tells a lot about what is in the subconscious mind, the part that runs us all the time more than what we think with our conscious minds.
So that movie that talks about evil, the stories children are so quietly engrossed in, that hurtful remark someone made or many other things we think mean nothing to us, becomes part of us, too. When God gives us the Holy Spirit to listen to, it is a big help but we can override it by putting junk in our minds.
It is bad enough to let it in, but to speak it into the world is worse.