Who is the that person?

Aside from the whole plot, you may want to spend some time in developing the protagonist and other important characters in your writings, chiefly i am speaking of novels.  You could have some understanding in your writing of the protagonist, but maybe not enough.  Set aside a few pages and develop the protagonists personality.  If he/she is a strange mysterious figure that is always surprising people, or a strong, bullheaded person who cares nothing for being sly in anything they do.
It can take a while before you find out how to do this, but once it's done you and others will understand that character better.  And don't let your antagonist be weak in character, no.  Writing about the "Bad Guy" is one of my favorite things actually.