Nicole Kidman did a beautiful job playing Virginia Wolf in "The hours" even though they look nothing alike.

Charlize Theron  had a pretty impressive make up job done on her and gained 30 pounds to play Aileen Wuornos in "monster"

Jeffry Write did an awesome job playing Basquit in a film about the  artist's life.

I think it's mainly acting skill and good hair and make up. I don't know how great of an actress Keira Nightly is,  but at least she's got a British accent, so that's a good start...