The agent subject is the classic doer of the action. An agent subject is an animate being that acts deliberately, with intent. Most speakers consider the agent the most typical subject.
Agent subjects can co-occur with either transitive verbs:
Catherine's boss fired her.
Fred threw the frisbee.
Joan built a birdhouse.
or intransitive verb:
The little boy yelled.
Those kids are whispering.
My niece smiled.
