Dialogue Drift

So, you're writing along, knowing where you need to go in a certain conversation and someone says something that's VERY important...but not part of the original plan. What do you do? Usually, this means a cut and paste deal for me while figuring out which bit should come first and how to yank the dialogue back to where it needs to be for set up for the next scene...and, yes, that takes work...or I try a couple of different ways and only hope that when I'm done, the flow seems natural...

May all your dialogue be smooth and brilliant today.
Robin