Sin never seems to just burst through the door waving a flag, blowing on a bugle and screaming "COME ON FOLLOW ME!"  Instead it creeps in slowly, at least in a believer's life.  Like David on the roof top, maybe we look just a little too long, or take a second glance.  Someone flirts and we flirt back.  Someone gossips or speaks ill of someone, and instead of shutting it down by changing the subject or walking away we join in and make an unkind remark.