Truth is stranger than fiction and there's a good reason for that. While life doesn't always make sense and people do some strange things, often for no apparent reason, whatever they did is a fact and we have no choice but to accept it. Because it's true.
It doesn't work that way in fiction. In fiction, things need to make sense and characters need to behave in ways that are in keeping with their personality, abilities and history. We readers expect the characters in the novels we read to be believable. We need to know them well enough to understand and accept their actions as being something they could or would do given who they are and the circumstances they’re in. Even if their actions are bizarre. Especially if their actions are bizarre.
What happens when I'm reading a novel and one of the characters does something totally “out of character,” something that doesn't ring true given what I know about him or her? What that tells me is that the author hasn’t fully developed the character and/or the author hasn't given me enough background information. When that happens, I stop caring about the character. And, I stop trusting the author. From that point on, if I bother to finish reading the book, I've lost interest because the character isn't believable.
Have you ever lost interest in a book because one of the characters was unbelievable? (Please don't mention specific titles.)