I read this wonderful book last night by Elizabeth Jennings titled Homecoming. It has a wonderful cast of characters who appear to be crazy at first glance. But as the reader becomes more involved with the story, they realize the characters' sanity (or lack of sanity) is a matter of perspective.
The same is true of the people around us in real life. Mostly their actions are logical and sane from their viewpoint if not from ours. When we know their motivations and experiences, then we more easily understand why they do the things they do.
When we writers create our cast of characters, we are constrained by our characters' motivations. If we don't know why they act the way they do, then how can we perceive whether their actions are true or false? How can we decide whether they are sane or simply marching to their own drummer?
Perspective is everything.