Interviewers, the good ones, have various ways to push an applicant's buttons.  Some of these include the "What If" question,i n which the interviewer sets up a hypothetical scenario and asks for an immediate solution.  He may be casual, but don't you be.  Pause, don't snap thefirst idea in your head.  Consider for a few moments (a good answer is more essential than speed.  Don't be afraid to add second and third possible options.   The interviewer may also try the classic "Stress Question", e.g. Why did you handle this this way in your old job, I don't think that's what we're looking for here."  the best preparation for this is to recognize the query for what it is - an attempt to spark a flare up in you.