Most of the time I am blocked I find its because of a cognitive dissonance. I am asking my characters to do something that I have hard baked in them the propensity NOT to do. So, I back up to discover how it is that I can make sense of something that my character logically would not do, i.e., a woman who is a stoic breaks down and cries in front of a man she is attracted to. She would NEVER do this. However if I have created a backstory establishing that she ALWAYS cries when she hears Waltz of the Flowers from the Nutcracker Suite and that's what is playing in the background when she and this man engage, there is an ambiguity regarding the reason for the tears. I do this exercise with regard to people, places and plot lines.