Host: "I wanna ask something I know a lot of people want to know. What is it like for you to hear the President of the United States call your Dad a liar?"
"Ahh You know, ah, Jump out of your seat and rush down to the debate stage and take a swing at him. But you know you can't do that because, well, first because there's a lot of secret service agents between you and him but also because this is the nature of the process."
Seems possible it was the moment when the President put this expression on Mitt's face.