I'm really new to this whole big-kid job thing.... so what do you say if you get a job offer and you either
A) want to be sure this job is for you. (or make sure you don't get better offers in the very near future

) In other words, you are interested in the position, but you don't want to give an immediate yes or no.
B) you don't want the position or it wasn't what you expected it would be after the interview