Are there any rules or other metrics that help determine if a role is considered a leading one or a supporting role? Or is it more like a gut feeling? Clearly, the choice is obvious in many movies such as Terminator or Pitch Black. But how do they determine the lead when there are several "main" characters to choose from?
For example: Lord of the Rings had Elijah Wood (Frodo), Ian McKellen (Gandalf), and Viggo Mortensen (Aragorn). Or Pulp Fiction's John Travolta (Vincent), Samuel L. Jackson (Jules) & Bruce Willis (Butch). How did they determine who was the leading role?
(Not sure what tags to put on this one)