Actors learn how to be actors in acting schools. Sometimes it may occur naturally but a good acting school is important. An acting school is where actors perfect their natural acting talent. It is also where they can find temporary acting jobs and get to know some good casting agencies.
The cast of Learn the Difference - 2011 includes: India Wilson
The cast of Learn to Dream - 2010 includes: Bryan Scuteri as Narrator
The cast of Look and Learn - 1982 includes: Takatsugu Kaizaki as himself
The cast of Learn a Language - 2011 includes: Julian Martsenkov as The Man
The cast of Live and Learn - 1920 includes: Charley Chase Marie Mosquini
The cast of And Gladly Would He Learn - 1964 includes: John Stockbridge as Narrator
The cast of Learn Self Defense - 2005 includes: Mark Cook as Narrator
So that the actors can learn their lines. Obviously some films don't have scripts and the actors improvise their diologue.
The cast of The More You Learn - 2010 includes: John Gard as Husband Theodore Horrocks as Employee
The cast of I Want to Learn How to Fly Fish - 2009 includes: Brian Hightower
stage directions
The cast of Tom and Esther Learn Lessons - 2010 includes: Jenna Kruger as Esther