Karen is actually a cousin to Caitlin Blackwood, the Scottish girl who played young Amelia Pond in the Doctor Who episodes "The Eleventh Hour", "The Big Bang", "Let's Kill Hitler" and "The God Complex". They had never met until they both showed up for the script table read, since Gillan's family had moved away before the younger girl was born!
In "The Time of the Angels", she assumed that Matt wasn't going to actually bite her hand when he's trying to convince her it wasn't made of stone. He did.
In "The Angels Take Manhattan", she originally said she didn't want to cry during her departure scene, saying that Amy was acting spontaneously and wasn't letting what she was doing sink in. However, when it came to do the scene, she simply couldn't stop crying.