What separates a really good stage or screen actor from those who come across as somehow wooden or fake is the simple fact that good actors don't act. The high school kid in the play acts the way he or she thinks (1) the character would act and (2) the way actors should act. The really good professional actor, instead of pretending to be like the assigned character, becomes that character, embodies that character. The wooden actor imitates another person while the good actor develops an imaginative identification with the assigned character and lives the role.
Perhaps one way to convey this idea is to have you look at two versions of a song. Go to YouTube and watch and listen to Jessica Simpson and Nick Lachey sing "A Whole New World" from Disney's Aladdin. Then listen to the same song by Brad Kane and Lea Salonga.
Here are some links:
After you watch the faces listen to the singing, tell me who gets it and who doesn't? Which pair has become the characters and which pair is just singing a song? Who is genuine--no scratch that--who demonstrates the passion and feeling of the dramatic situation?
So, here's my Acting in One Lesson advice: Stop acting like the character, stop pretending to be the character, stop talking as the character would, and instead, let yourself become that character. And paradoxically enough, to do that you have to reference yourself. Don't think, How would this character say that, but How would I say that if I were in that situation and had those values or options? (For a bad character, for example, ask, What would I say if I were a liar or confidence artist or thug? If you want to be natural, you need to imitate the most natural person you know--yourself.
In word then: Aspiring actors: You can become a good actor by becoming the character by putting yourself into the character in the situation. Connect yourself to the dramatic situation, not just to the dialog. I hope that makes sense.