She's a fairly well-known actress from Emmerdale and Waterloo Road, and her casting was announced some time ago and publicised widely.
That's kind of my point - I knew all along that a person called Jenna Louise Coleman is new companion Clara Oswin, I have seen pics of her filming, and I still didn't recognise her... so what chance has the casual viewer got to get what's such a "twist" (as even the Radio Times promised) about her? Until Xmas anyway...