Earlier in the week, the New York Times published an article outlining how the presidential candidates prepared for the debate. The findings were exactly what anyone who has been following the debate would expect.
Hillary Clinton rehearsed her answers to questions, looking to keep them under the given two minutes, and researching to get her facts right. Not only was Clinton practicing with a Donald Trump stand in, but her team told the New York Times that she had multiple people playing Trump at a time. Clinton did all the things a traditional candidate would do to prepare for the first presidential debate.
Trump went, as is expected, a more unconventional route. His campaign team told the Times they practiced keeping the conversation to bigger issues rather than getting caught up in the facts of an issue, since Trump has gotten them wrong in the past. He is also apparently sticking to his primary debate strategy of aggression and insults. In his mock debates, Trump has not worked to keep his answers timed.
Clinton prepped to take insults and to be questioned about her emails, as well as call Trump out on his inaccuracies. Trump readied himself to call Clinton out on her emails if she called him out on fact checking.
Supposedly, Trump's main way to prepare for the debate was to watch recordings of Clinton and himself to find weaknesses and strengths.
The New York Times also expressed concern over Trump's stamina, as the 90- minute debate involved only two people and Trump is used to being on an over-packed stage. In those debates Trump had the opportunity to rest and jump in when he needed to, but in the one-on-one, his berating style may be too much.
The article mostly said what is to be expected from both of the candidates: both of them confident, Clinton because she wants to be the best prepared, and Trump because he thinks he cannot lose.