There are two basic types of people who get leading roles in movies: "
actors" and "Hollywood personalities."
Actor implies some training and skill at a craft. They exhibit an ability to put aside their own personalities traits and assume those of another person; they can exhibit a range of emotions and character traits convincingly. They fill a written role, rather than having roles written for them. Marlon Brando, Glenn Close, Anthony Hopkins, Emma Thompson are prime examples of great actors and actresses.
Hollywood personalities are people who do not have the same training/skill as actors and thus have a very limited range of possible roles. However, they continue to get movie roles because there is a demand for their specific personality.
Adam Sandler is the classic example of a Hollywood personality, as are most other
SNL alumni. They have roles (and often entire movies) written specifically for them; nobody else could play the part, and they often could not play any other part.
It is sometimes possible to mistake Hollywood personalities for
typecast actors. For instance,
Bruce Willis is an actor, though he almost always plays "tough guy" action-hero roles (though not as much as of late with
The Story of Us and
The Sixth Sense); same with
Nicholas Cage and
Steve Martin. The sure sign of a true
typecast actor versus a Hollywood personality is that the typecast actor succeeds at a
breakout role, like
Jim Carrey in
The Truman Show, whereas Hollywood personalities fail (often miserably), like
Keanu Reeves in
A Midsummer Night's Dream.