I have a question about acting.?

How do actors do this? Lets say for example.. They get a role for a movie. The character they're playing has a very gloomy,brooding,strange personality. They keep to themselves and don't really talk to people. How are you actors supposed to know how to become this "character"? How do they know what type of facial expressions to make at certain scenes,etc? When I was watching the movie Twilight, the main character (Edward) has a very ominous personality. He seems very withdrawn and seems very mysterious and always has this painful look on his face. But the actual actor who plays him is actually quite outgoing in real life. How did he learn to become this character so well and learn how to make these facial expressions,mannerisms?