There are times in our lives when we experience events for which we're not quite prepared. An hour, a day, or a week later we have that smack-the-head moment when we think, "What I should have said was--"
That's the nice thing about being a writer. Even though we can't go back in time and rewrite reality, we certainly can recreate that moment in our fiction and make ourselves--um, er--make our protagonist say the things we should have said. We can cut our antagonist to the bone, prevent our lover from walking out the door, or get that promotion.