Steven: I’ll draw your attention to Irene’s Evil Hair. She’s currently (the couch scene at 221B) got her Nice Hair because she’s being nice. But when she goes after this scene to see them on the plane, she’s reached for that concealed switch at the nape of her neck that just turns her hair evil again.
Steven: That happens instantly. She’s able to do that within minutes.
Mark: It’s actually sort of like a superpower isn’t it?
[ Sherlock Series 2 DVD commentary]