I was pleased to see Trump so restrained during the debate, but I do think it was a calculated plan to just "let Joe be Joe" and become his own worst enemy. Trump was wise to sit back and only speak when it was his turn.
I doubt that Trump would have been as passive if another person were debating him that night, so people shouldn't assume that they would have gotten the Biden "Trump." They would have gotten the "Let Trump be Trump" Trump.
-PJ
And thanks to CNN because of their stupid rules. Oh, and there's no audience. So that helped Trump because he did not feel like he had to prove the audience anything.