Build a natural character
The most important thing to start withs is observation. Watch other people. Take some time off and stay at the mall, of in a restaurant, observing other people. One day I observed a homeless woman selling journals. It was amazing how kind she was to everyone that passed her. I saw how she was always bending a bit to the next person, her head slightly upwards. Always a little smile. She stayed on her spot, sometimes moved one or two steps but never more. She repeatatly put her hair, getting in her eyes because of the wind, behind her ear while closing her eyes for a sec. Intriging. Her clothes were in dark shades, but I could see a colourfull shirt underneeth. When someone bought a newspaper she bowed, as if she got applausse from a big audience. Getting closer I could her her breathing, quick, like nervous. Just by observing her I learned a lot. It didn't make me a creeper, I just got to see a bit of how this woman goes about in her merry (or not so merry) lives. Observing makes you notice the variety of people. You will learn about the quirks and mannerisms which really define them. And that is interesting. See what they do with their hands, their eyes. Make note of any particular characteristics which strike you. You might discover if someone has a particular tick, a striking facial expression, or a jump in their step. Keep studying; the more you watch, the more you learn. If possible listen to the way they speak. If anything stand in front of a mirror and practice various poses and expressions and think about how they make you feel and how you would feel seeing someone else doing it.
While building a character always realizethat we are constantly assessing one another, moods, status, reputation, esteem (self confidence), intelligence. In order to make the audience believe and love or hate the character, you as an actor have to complete every part of human behaviour, emotion and psychology to make a complete human being.
It is not always easy, but this is the way actors create their characters. Knowing everything inside out about the character, his reasons to be in the given circomstances, makes the audience believe. Even if in another play of movie the same actor is a totally different character. That's why it's an art.
Our tip to you is to observe and than use your observations to create the character. I got to play the journal selling woman for an audition to get in my actingschool. And that got me there. Thank you lady, and sorry I never asked your name.
Laios Egri's books are the best instrument if you want to learn more! I strongly advice everyone to buy his books!