“Idjits!” —Bobby Singer, SUPERNATURAL.
In the last week, I’ve read three books that depended on stupid main characters surrounded by people in the know. I’m not talking characters who were mentally impaired in some way. These characters were intelligent enough, but they were in situations where they were totally ignorant and everyone else knew and understood what was going on. Heck, one book’s pet cat was smarter than its humans. Sometimes, the main characters even refused to accept what the knowlegable characters knew despite evidence to the contrary.
I’ve always preferred intelligent characters, but these characters bothered me beyond their behavior. In each book, the constant need for explanations, protection by the more knowledgable characters, and the utter incompetence sucked the forward motion and interest right out of the book.
Your main character should be active, not passive, in personality, plot choices, and forward motion, and stupid will suck the active right out of every element of the book.
This doesn’t mean you should write a good-at-everything “Mary Sue” or a Sherlock Holmes character who understands everything, but you should have a character who has enough knowledge to move forward to gain more knowledge or information as the plot moves forward. And, by no means, should you ever use stupidity as a major plot device through a book.