Unlike the Primary debates Donald Trump will have to deal with only one person. Where he's been accustomed to fading into the background at times with his stage performances with lil Marco, Lying Ted, sleepy Ben , etc. he now has to sustain 90 minutes on on-stage presence. Whereas he's accustomed to dealing with big-picture stance - like I know how to make America work again but I don't want to reveal my plans now, etc. - he now has to deal with pointed questions of issues. Whereas he was playing to a Republican audience in the Primary debates, he now has to satisfy the undecided Independents and those wary of both himself and Hillary.
The real campaign starts tomorrow because Democratic voters and Republican voters ho made up their minds are not the ones for the taking here. It's the Independents and Undecideds.
Trump has played off-Broadway and off-off-Broadway well. How will he do under the bright lights of the great White Way. We know how Hillary does on a stage with one-on-one contenders (Barack in 2008 and Bernie in 2016). She knows knowledge of the issues takes a background to likability. She's had some experience dealing with that. Does Trump know how to deal with those he called fat pigs, not a hero, bleeding from you-know-where, etc?