Study your profession from as many perspectives as possible. Watch actors in different media, including television shows, theater and film. Watch them in a different way from a person who is just enjoying the story. Leave the story in the background and observe the actors instead. Form your own judgments about the type of acting you like and the kind you do not. Observe the character actors, particularly, as they often give understated, naturalistic performances.
Develop your acting skills. Join a drama group, attend drama school or hire the services of a reputable acting coach. Listen to the advice you are given and pick up as many acting tips as you can. Spend time with your fellow performers. Exchange views about their acting and yours. Talk, in particular, to those actors whose style you admire.
Observe people all the time. Notice the way they speak, their posture and how they move. Look beyond the superficial, to the expressions on their faces and how they betray feelings in a nonverbal way, perhaps by swinging a foot, or looking nervously away. Call on these observations when you attend auditions. Attend a variety of auditions and resolve to stay positive in the face of the almost inevitable rejections that actors accumulate along the way.
Work on other aspects of acting so you can free yourself to act in a naturalistic way. Develop your memory, so that you can learn your lines with relative ease. Improve your self-discipline all the time, as actors have to be highly disciplined.