Have you ever come across characters who are too stupid to live? A few of these are to the point where the reader is screaming "Stop being an idiot. You're going to get yourself killed!"
It isn't so bad except when the character happens to be the hero or the villain. (I'll admit I've written a few of these, but no one will ever see those books.) Most of the time I find this idiocy occurs when the character is one dimensional. Since I realized this I try to know my characters, even the most insignificant person has there own story, their own background. Most of this never gets used in the story, but I know it, and knowing it makes those idiots gain more depth and become real people.