"Smiling at me kind of devilishly," Falletta said.
She gave him her baton and stepped aside.
Gesturing exuberantly, the president led the orchestra during part of its performance of "Stars and Stripes Forever."
"We didn't expect him to know the score so well," Falletta said afterward. "He was not shy about conducting at all. He conducted with a great deal of panache."
That was the music played for Bush's exit after his speech at a ceremony commemorating the founding 400 years ago of Jamestown, America's first permanent English settlement.
Just before the music ended, Bush turned to Falletta, who stood on a step below him, kissed the top of her head and left without saying a word.
The 400-strong orchestra was made up of about 50 members of the Virginia Symphony, plus musicians from youth orchestras around the country. The switch in conductors was impromptu, said Falletta, the symphony's music director.
"I think he may have just been seized by the desire to conduct the orchestra," she said.
Falletta did not spot Bush until alerted by a musician.
"I'm embarrassed now that I didn't notice him, but I was just thinking of the music," she said.
Bush stepped onto the podium and took over.
The musicians were impressed by how musical Bush was, Falletta said.
"He was cueing the brass, he was cueing the percussion, he kept the tempo going," she said.
Bill Fearnside, a violinist with the Virginia Symphony, put down his instrument and picked up a camera to record the moment.
"It was a little shocking, but it was fun," Fearnside said.