Thinking type Question!

Lets say youre trying to sell something to someone. Or just an important conversation. You have to ad lib (think on your feet). Maybe you get an Objection.
Do you say to yourself quietly and quickly "in your minds eye" the exact worded response before you say it out loud?
Remember that you need to have a smooth conversation that sounds natural so you cant pause too long.
Or do you just start talking and hope the words come to your brain and out of your mouth as you are speaking?
Remember, the person you are talking to is not your friend and you only have one shot to accurately express your self.