Tone and body language at the Harris-Trump debate speak as loudly as words

As their high-stakes debate in Philadelphia commenced on Tuesday night, Vice-President Kamala Harris walked across the stage, manoeuvered around a lectern and stuck out her hand in greeting.

"Kamala Harris," she said to former president Donald Trump by way of introduction.

Meeting him for the first time in person, Harris forced her opponent to shake hands, something he and her predecessor as the Democratic presidential nominee, Joe Biden, did not do in June.

It was one of many moments at the debate in which the two candidates used body language, tone and demeanour instead of talking points or policy positions to do battle.

"It was a power move," said body language expert Mark Bowden, referring to Harris' pursuit of Trump for a handshake. "I don't think he was expecting that."

The vice-president maintained an expressive face throughout the evening, raising her eyebrows, laughing, and looking on as if in disbelief as Trump spoke, moments that were quickly snapped and recirculated by her fans on social media. At times she looked at the Republican nominee quizzically and remarked that his comments were "not true".

Trump mostly kept a serious expression on his face, while sometimes smiling with pursed lips or shaking his head. He accused Harris of mistruths as well. "There she goes again. It's a lie," he said at one point.

Throughout the debate, both candidates raised their voices. Harris, 59, frequently turned towards her opponent as she spoke, at one point appearing to scold him. Trump, 78, mostly looked straight ahead.

"Their approaches were very different," said behavioural scientist Abbie Marono. "He didn't address her; he didn't orientate toward her when he was speaking ... She was a little antagonistic towards him, but she was also very emotionally responsive towards him."

Trump sought at one point to turn the table on Harris when she interjected, telling her to wait because he was talking. "Does that sound familiar?" he asked, in an apparent reference to her 2020 debate with then vice-presidential candidate Mike Pence, whom she rebuked for doing the same.

Harris often returned to talking points, repeating policy proposals about housing and small businesses and calling for the country to "turn the page" on Trump.

"It's obvious that she's memorised things," said Lillian Glass, also a body language and communications expert, who said she thought Harris came across as inauthentic. "She speaks in staccato."

At the end of the debate, both candidates, dressed in dark suits and American flag pins, walked offstage. They did not shake hands again.