I would say knowing how formally/otherwise to dress. You've either got the chops or you don't, you can feel out the people you're speaking to during the interview to gauge how whether you can crack a joke or not. But turning up either woefully under or over dressed just throws you off your stroke at the first hurdle. I always explicitly ask what is expected of me ahead of an interview (with the bonus effect that it they want me to dress up for work, I know not even to bother with the interview!).
