body: I watched many American movies in which they tell about a person who writes a personal real life story of his best friend without his acknowledgement.
  
  
  And the movies said that person is wrong and in some cases his best friend can sue him to the court.
  
  
  Is writing a real-life story about a person is illegal in USA? 
  
  
  If so, does it matter if that person changes the names of the characters in the story, so that no-one can have an evidence that he was writing about his best friend?
  
  
  
title: Is writing a real-life story about a person is illegal in USA?