Becoming the character you are playing might work for some, but for me, it doesn't. I always maintain a gap between myself and my character because if I will go so deep into it, it will get difficult for me to come back. You should work towards understanding the psyche of your character and then play it.
If you normally go quiet when you get angry, you may not relate to a character that reacts very differently. But if you see someone internalising the anger like you do, you will immediately identify with him or her. It's these small things that penetrate your psyche and make you relate at a deeper level.
Whenever I read a script or sign a film, I don't see whether he is a bad guy or a good guy. I see how much the character is contributing to the story? How much is the importance of the character in taking the story forward? And what new I would be able to learn and what new I would be able to try in that?