bad interview

Perhaps you’re late to the interview, you haven’t had a chance to review the candidate’s resume, or you call him “Dan” instead of “Derek”. This is a senior level position, Derek is a seasoned professional and you can tell he’s annoyed.
The best thing you can do is apologize and be upfront. Say something along the lines of, “Please excuse my tardiness, you’ve caught me on a bad day. Would you like to reschedule or are you okay to continue?”