Look for themes of trust, poise in tonight's debate

WASHINGTON -- The most telling moments in presidential debates often come out of the blue -- an offhand remark or unrehearsed gesture that helps reveal the essence of a candidate who's already been poked, prodded and inspected for years.

Hillary Clinton and Donald Trump have competing missions heading into tonight's leadoff debate of the general-election campaign: Hers to overcome the trust questions that have bedeviled her for decades; his to convince voters he has the good judgment and restraint required of a president.

Plenty of subtexts will play out as well over 90 minutes before an estimated audience of 75 million or more television viewers -- an outsized share of them disenchanted with both candidates.

Some things to watch for tonight:

Clinton vs. man of mystery

Donald Trump

Just who will show up to debate Clinton? Will it be the say-anything Trump who roiled the primary debates by dishing out a stream of insults and provocations? Or the rein-it-in Trump who's been trying of late to demonstrate he has the maturity and measured temperament to be president?

One possible clue: Watch to see whether Trump trots out the "crooked Hillary" nickname or puts it on ice for 90 minutes.

Take a deep breath

Expect Clinton to try to goad Trump into losing control, perhaps by questioning the size of his wealth and the success of his businesses or by highlighting his past incendiary statements about minorities, women and others. Trump is promising to "stay cool." But 90 minutes could be a long time for the master of improv and theatrics to hew to a script.

Policy pitfalls

Both candidates have policy gaps to fill in and changes in position to explain. At its best, the debate could help flesh out details of both candidates' platforms, highlighting similarities and differences. There are pitfalls here for Trump in particular: Weak on policy, he's vulnerable to slip-ups that could feed into the not-ready-to-govern line that Clinton is pushing. Trump has been studying up: You can bet he now knows what the nuclear triad is. (During the primary debates, he seemed not to understand it represents weapons in silos, submarines and bombers.)

Those 'damn emails'

Clinton largely got a pass during the Democratic primary debates on her use of a private email system when she was secretary of state. Primary rival Bernie Sanders, in their first debate, did Clinton a favor when he declared that "people are sick and tired of hearing about your damn emails." Don't expect Trump to cut Clinton a similar break. She also has more to answer for since the FBI concluded she was "extremely careless" in her handling of classified material in the emails. Clinton has been struggling to find an effective explanation: Now would be a good time for her to nail it.

Push-ups, anyone?

They can't exactly drop to the floor for a one-armed pushups contest. But look for both candidates to more subtly project health and stability. After her much-publicized coughing fits and recent bout of pneumonia, Clinton will be out to show she's got the strength and stamina the White House job demands. As for Trump, critics have speculated he has any number of psychiatric disorders. It would be a good time to show a level head and solid grounding.

Pointers and pincers

He shrugs. She bobs her head. He waves his arms. She pinches her thumb and index finger. Every wink, nod and fidget tonight will be analyzed for silent messages that can speak volumes. President George H.W. Bush caught grief for stealing a look at his watch during a 1992 debate. Al Gore's audible sighs in a 2000 debate were seen as discourteous to George W. Bush.

Factivism

The candidates won't be the only ones under the microscope. Moderator Lester Holt of NBC News will be under enormous pressure to maintain control and act as an objective referee. In the leadup to the debate, Trump maintained it would be improper for Holt to try to fact-check the candidates' statements in real time. Clinton spokesman Brian Fallon tweeted if debate moderators don't fact-check the candidates, "it is an unfair advantage to Trump, who is a congenital liar."

Gender dynamics

Gender politics will be afoot in the first general-election debate to feature a woman. Trump had trouble navigating this terrain in the primaries, when he tried to back away from a derogatory comment about rival Carly Fiorina's looks by declaring in one debate she had a "beautiful face." Clinton will be ready. She said earlier this year: "I have a lot of experience dealing with men who sometimes get off the reservation in the way they behave and how they speak."

What to wear

Call it frivolous, but people will check out what the candidates wear, especially Clinton. When comic Zach Galifianakis recently asked Clinton what she was going to wear, Clinton said she had no idea and scolded him for "this thing called the double standard." As for what Trump will wear, Clinton said: "I assume he'll wear that red power tie." Alluding to questions about whether Trump is a racist, Galifianakis replied: "Or maybe a white power tie."

Post-mortem

Even if you watch the whole debate, its impact may not be completely clear until the post-debate pontificating plays out. The analysis and selected clips that are highlighted after the debate can have a big influence on the millions of people who didn't tune in -- or who watched Monday Night Football instead. And why wait for the debate to end? Your Twitter feed will be filled with significant moments before you've had time to digest them.