We really need to know the context to give good definitions.  I constantly fall into the trap of giving the first definition that comes to mind only to learn later that the poster had something entirely different in mind.  

After giving the above meaning of the term a few minutes ago, it struck me that there are another very unrelated meaning* and the one below which is related but much more clinical and precise**.

*You can read a story and "act out" the parts of the characters as if you were on a stage.  Let's say that you read a Sherlock Holmes mystery.  You and your friends can pretend to be them, each friend "acting out" a character from the story.

** "psychoanalysis : to express (repressed or unconscious impulses) directly in overt behavior without awareness or insight especially during psychoanalytic investigation"  (Merriam-Webster)