From my experience you never know which person you are going to get from me in an interview. If I'm swamped that day and just did a deployment or something then I'm probably already spent or preoccupied mentally by the time the interview rolls around that day. That was my experience when interviewing at Facebook earlier this year. I generally will go on interviews to at least learn what I don't know or what I don't know well enough to do in my sleep. However, many times its just the luck of the draw that day as to which version of me you'll get.