Dancing with Juliet...and Romeo?

A believable character first of all, must have a believable ‘why’ and a ‘how’, and consistent actions, words and emotions to support the ‘who’. For instance, if a character is to fall in love with the enemy, the reader will believe in the character and his/her actions as long as the plot explains how and why this happens. We have all read Romeo or Juliet, or at least seen the movie (or heard about what it’s about, I hope), Juliet is a believable character because we understand the circumstances around her life, the how. We are led to understand how she falls in love with the enemy, and we ‘believe’ in the character, the how makes sense. At the end, we believe that she would take her life for Romeo, and when in fact she does, we believe it, its it consistent with the plot, and because we understand why. We believe in the characters as long as there is a convincing how and why, and the characters actions, words and feelings are supported by the plot.
From my perspective, readers believe in the plot out of faith. The plot in fiction can take place in a galaxy which doesn’t even exist, or in an underground Eden or anywhere, anytime. It can have circumstances which are illogical or out of the reach of reality. A reader picks up a book, is interested enough by the characters, the theme, something, then keeps on reading to gradually uncover the plot and begins to peel it like an onion. At that point, the reader will keep on reading as long as the plot moves along with the character, dances with him/her, and they move together through conflict and resolution, action and reaction. If, however, at some point, the plot and the characters become strangers, or if they are so different that they seem to be two different stories, then the plot may no longer be believable, or possibly, vice-versa.