The protagonist is the prime-mover of the story, the one who drives the story forward to it's conclusion.
Note that the Protagonist doesn't always coincide with the Main Character. For instance in a Fish Out of Water story the Main Character steps into a pre-existing story with it's own Protagonist.
Also in Multi-strand Narratives each story fragment may have it's own protagonist - there may not be one overall protagonist. In these style of movies the key articulating idea for the movie might be a theme or a corporate body (such as a town) rather than a individual.
In Dramatica:
Action Characteristic: Pursues the goal. The traditional Protagonist is the driver of the story: the one who forces the action.
Decision Characteristic: Urges the other characters to consider the necessity of achieving the goal
