The split screen told the story at the first 2016 presidential debate, and it was not kind to Donald J. Trump.

At the right was his opponent, Hillary Clinton, a practiced one-on-one debater, who held long, studied gazes on her opponent, delivered calibrated attacks and turned to the audience to smile incredulously at his responses.

At the left was Mr. Trump, the volatile presence who alpha-dogged a season of Republican debates. Now he grimaced, squinted, nodded, pursed his lips, sniffed, huffed and interrupted, becoming, over the night, an agitated man in a box.

This seemed to be by Mrs. Clinton’s design. She had reportedly prepared to face two different versions of her opponent. There might be a reserved Mr. Trump, ready to reassure voters about his self-control. Or there might be the raging, ranting Mr. Trump of the primaries.