May be I am a little overconfident, but I always go into an interview and treat it as a formality to getting the job. By the time I'm in front of the whiteboard, I've already spoken to the recruiter and know what they are looking for, seen the job req to know what technologies they care about, and I've had a phone screen. Why wouldn't I be confident?
Another interviewer told me at the end of the interview to submit an application and they can get an offer emailed to me by the end of the day. We did a collaborative white board design session and spent a lot of time just talking.
Either all of the job offers I've gotten have been mostly just discussions or I talk tech so much I can't tell the difference between an interview and a typical day at the office.