How do actors make their faces look different while in different characters?

I'm not talking about makeup, special effects, lighting, etc. For example, Gary Oldman can be identified easily by seeing him. But, his face looks one way in The Professional, another way in The Dark Knight, and (really) another way in The Book of Eli. I noticed Chrisitan Bale doing this in Rescue Dawn, Reign of Fire, and The Dark Knight. His mouth is different, and his face is like held a certain way. You know who it is, but their faces in general are like being held a certain way. Some actors don't do that really and are still good at being different characters. For example, Denzel Washington looks the same all the time. So does Morgan Freeman, etc. Just curious. Has anyone else noticed this?