good character actors are rare in RPG's in my experience because character acting while more rewarding is often very taxing. Still my best experiences have been when I shoved me aside and channeled my character. Its why character creation for me is more than a series of numbers and why sometimes I realize that even though I created a great character, I cant continue to play them.
Character acting for me started with Vampire the RPG where I played a gangrel biker chick who had no idea she was a vampire. She would have these fits which she couldnt remember and then she'd wake up covered in blood and terrified.
One day, she was caught by the cops and well, thats when she snapped,... just snapped and I did some horrible horrible things to them before they died. It worked, really well and moved my player from the scared she was a serial killer camp to the aware she was a supernatural being camp. When I looked up after a detailed and dramatic hand to hand fight all the other players were staring at me, in a not so good way. In a is that my mom calling kind of way. But I had been planning this outburst for a while and I had to stop and explain to them why I had just become a real monster.
It was a good session. Beth, the storytellers girlfriend later did much the same thing with her characters all of who were heavily character acted. I'd tell you what she did in character that had us going check please, but this is a PG board. Anyway my point is that anyone can play a collection of numbers, and can even make it fun, but I really like making rounded characters for my edification.