This is why I don't want him there. Y'all are confusing Johnson with the Libertarian Party, but he's not that far off from a hijacker. The guy actually had a pretty contested primary in the sense that actual Libertarians divert a decent bit from Johnson. Sure, he's moved since he was a Republican, but not so much that he matches the insanity that his party represents. If you try to pin him down on the anarchist stuff that the Party believes, he can pretty easily just go, "No, I don't believe that. Some people in my party believe that, but it's not my position."
And I'd definitely argue that unless the event is super scripted, Trump can't last very long in the public eye without fucking up badly. The longer he has to stand there and come up with answers, the better for Clinton. I agree with Crab that her best move is to Bernie him and just ignore him; she can use her time to sound presidential as fuck and just spout policy the whole time. It's boring, but we now have polling that implies people may be looking for boring and safe right now. Let Trump have his spotlight and start rambling. He can probably think up enough canned statements to last for the first half, but since his prep is apparently not very thorough at all, I doubt he can do much more without going off the cuff.
Pretend you're Trump's campaign manager. Is the phrase "Trump is going off the cuff" a phrase that makes you feel hopeful? Or do you flinch and suppress a panic attack?