WASHINGTON — The most telling moments in presidential debates often come out of the blue — an offhand remark or unrehearsed gesture that helps to 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 that he has the good judgment and restraint required of a president.

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

The debate begins at 9 p.m. Some things to watch for:

[naviga:h3]CLINTON versus INTERNATIONAL MAN OF MYSTERY[/naviga:h3]

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 to demonstrate of late that 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.

[naviga:h3]TAKE A DEEP BREATH[/naviga:h3]

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.

[naviga:h3]POLICY PITFALLS[/naviga:h3]

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 that it represents weapons in silos, submarines and bombers.)

[naviga:h3]THOSE ‘DAMN E-MAILS’[/naviga:h3]

Clinton largely got a pass during the Democratic primary debates on her use of a private e-mail 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 e-mails.” Don’t expect Trump to cut Clinton a similar break. She also has more to answer for since the FBI concluded that she was “extremely careless” in her handling of classified material in the e-mails. Clinton has been struggling to find an effective explanation: Now would be a good time for her to nail it.

[naviga:h3]PUSH-UPS ANYONE?[/naviga:h3]

They can’t exactly drop to the floor for a one-armed push-ups 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.

[naviga:h3]POINTERS AND PINCERS[/naviga:h3]

He shrugs. She bobs her head. He waves his arms. She pinches her thumb and index finger. Every wink, nod and fidget 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.