Before you can truly become a character - you must first know yourself.

Most characters have descriptions written directly in a script by the playwright - "he is an exceedingly handsome youth, dressed in a t-shirt, dirty jeans and cowboy boots." or "a pretty young woman with anxious lines in her face, enters the bedroom..." or even "she enters dressed in business clothes and carrying a briefcase.". A character description may go on to state the age of the character, an accent (if they have one), what they're wearing or how they enter the scene.  These character descriptions however, usually, do not state how the character walks, what they are thinking as they enter a scene, any bad habits or compulsions they may have or mannerisms they display.  It is these that take an actor from simply 'acting' to 'creating a life'.

When I am directing a play I make it very clear to my actors that, before any character notes are given, they will receive notes on themselves as actors.  During the first read-through of the script as a group and the first few rehearsals, I take note of my actors' mannerisms.  I play close attention to who they are. Do they sniffle because they have sinus issues? Do they tuck their hair behind their ear every other line? How do their eyebrows move when they are trying to think? Do they sigh during pauses? Are they a fast or slow speaker? Do they rock back and forth or side to side? Are their feet planted or do they like to travel about? Do they grind their teeth when they're angry? Do they fiddle with their hands, chew their fingernails, crack their knuckles or twiddle their thumbs? I pay attention to the small details which make them human.  I then take these notes and give them to my actors for one reason: They need to know what they do as a human being so they can either use them or lose them to create another human being different from themselves.

As I said, I am very forward and honest with my actors and reassure them that the notes I give them are not to hurt their feelings or to point out flaws.  I give them these notes because I want them to know what they do and say, how they walk and what the 'human being' that is them - looks, sounds and feels like to help them better understand who they are and what they need to do and/or change to become someone else.