Yeah its tough to shake an actors' face and voice once you've seen it. The actress who plays Cersei especially, she just does not deliver with nearly as much venom or impact as I'd imagined her. Nothing worse than a weak villain IMHO.
I think Lena Headey is awesome in the role. She has a restrained coldness that I think fits the character nicely.
After all, the character isn't necessarily evil, only self-serving.