She’s shown her biting wit (joking in 2008 that she was wearing an “asbestos” suit) and displayed a sympathetic, self-deprecating side. (In a memorable 2008 debate before the New Hampshire primary, the moderator asked Mrs. Clinton about her likability problem and she replied, “Well, that hurts my feelings,” to which Senator Barack Obama quipped, “You’re likable enough, Hillary.”) And she has perfected the art of playing the victim. (“Maybe we should ask Barack if he’s comfortable and needs another pillow,” she said in another 2008 primary debate, referring to a “Saturday Night Live” skit in which the debate moderators coddled Mr. Obama.)
All of those debates led to this one: Mrs. Clinton’s first foray onto the general election stage. The evening did not give Mrs. Clinton the zinger that enters the history books or the emotional, unplannable moment that would convince a skeptical electorate that she can be trusted, but it did give her the biggest venue yet to show her personality.
She displayed her preparedness and her quick-footedness, her grasp (and affection) for granular policy detail and her eagerness to throw flames and flip the script on her famously combative opponent.
“I have a feeling that by the end of this evening, I’m going to be blamed for everything that’s ever happened,” Mrs. Clinton said dryly Monday night after Mr. Trump said she had been fighting the Islamic State her entire adult life.
“Why not?” Mr. Trump replied tersely.
She showed a pugnacious side. “Well, Donald, I know you live in your own reality,” Mrs. Clinton said early on.
And she espoused detailed policy with the ease of a Pez dispenser.
When Mr. Trump delivered an impassioned, if unspecific, pitch to bring jobs back from Mexico and China, Mrs. Clinton explained her profit-sharing plan. When the conversation turned to foreign policy, she easily slipped into diplomat talk, vowing to “take out ISIS in Raqqa” to “end their claim of being a caliphate.”
Mrs. Clinton provided only passing references to herself, opening the evening by mentioning the second birthday of her granddaughter, Charlotte.
When Mr. Trump interrupted her, she showed flashes of the steely calm she displayed during more than eight hours of testimony to a Republican-led House panel. As Mr. Trump spoke, she perched a leg in a subtle curtsy and calmly looked on. When it was her turn, she needled Mr. Trump by calling him “Donald.”
Campaign aides told me the debate was one of the only formats that would allow voters — and the reporters who cover her — to catch an unfiltered glimpse of Mrs. Clinton, a candidate so cautious that even the most innocuous personal details (her favorite TV shows, for example) can seem overthought, and, as a result, come off as overwrought.
On Monday afternoon, as characteristically anxious campaign aides milled around the Hofstra University campus, I tried to pry out details of what Mrs. Clinton did the morning of the debate to deal with the stress.
For instance, I told Clinton aides that I did yoga to hip-hop music in a 90-degree room and drank two iced Red Eyes (cold brew with a shot of espresso). “Did Hillary walk her dogs? Did she do yoga? Do she and Bill binge on blueberry pancakes as a debate-morning tradition?” I asked.
“We’re not that kind of campaign,” a spokesman told me.
That makes the unfettered moments on the debate stage — a late return from the bathroom; a quick-witted response; a rare moment of self-reflection — important in trying to understand Mrs. Clinton.
She has said she does not have the retail political skills of her husband, former President Bill Clinton, who sat in the audience Monday night. And she does not deliver speeches with the sweeping oratory of President Obama. But ever since Hillary Rodham played a star role on her high school debate team, she has exceeded at the formal confrontation of the debate stage.
“Secretary Clinton, is that O.K.?” Mr. Trump asked Mrs. Clinton at the start of the debate. “I want you to be happy, that’s very important to me.”
And, Mrs. Clinton, flashing a wry grin, did look genuinely happy.
Her first general election debate, and the first with a female nominee of a major party onstage, provided Mrs. Clinton 90 minutes with no commercial breaks. That meant Mrs. Clinton, who had prepared for weeks, was on her own, without the advice of aides or time to ponder how a response would play with her widest audience yet.
On Monday, she tried to help an enormous TV audience better understand who she is, owning her image as the straight-A student who pulled an all-nighter at the library.
“I think Donald Trump just criticized me for preparing for this debate,” she said. “And yes, I did. You know what else I prepared for? I prepared to be president, and that’s a good thing.”
I’ve watched for years as Mrs. Clinton has walked the line between presenting herself as a public servant driven by her Methodist faith and the shrewd political maneuvering required to become the first woman to capture the Democratic nomination for president.
On Monday, Mrs. Clinton tried to make an affirmative case for her candidacy even as she displayed ease, and even enjoyment, referring to a vault of research on Mr. Trump that she had cemented to memory in weeks of practice sessions.
Mrs. Clinton told of people who said they had been bamboozled by Mr. Trump, pointing to an architect in the audience who said he never got paid for designing a clubhouse on Mr. Trump’s property. And when the topic of race came up, Mrs. Clinton seemed eager to cite an article she had read.
“Remember, Donald started his career back in 1973 being sued by the Justice Department for racial discrimination because he would not rent apartments in one of his developments to African-Americans,” Mrs. Clinton said.
Mr. Trump did not sweat the details. “We’ve spent $6 trillion in the Middle East, according to a report I just saw,” he said at one point. “Whether it’s six or five, but it looks like six, $6 trillion, in the Middle East.”
By the end of the debate, when Mr. Trump attacked Mrs. Clinton’s stamina, she seemed more energetic than ever, digging into the opposition research trove one last time. “This is a man who has called women pigs, slobs and dogs,” Mrs. Clinton said.
With her opponent on the ropes, she smirked. She was just getting started.