"Which one of these actions our character will follow depends on the kind of character he is. Thus a brilliant story plot is not enough to write a good screenplay because in itself it is not sufficient to tell us how the people in our story will "answer" the "questions" put to them by our plot line.
Authentic responses to plot questions require not random but appropriate answers by characters. Otherwise you'll hear your viewers complain out loud with that familiar "no way!"
Decide who your characters are while building up your story line. Otherwise your "answers" to plot "questions" will be pretty random, as is the case with a lot of bad movies out there."