Susan Marston, 58, a program manager from Boise, Idaho, said that unlike some longtime Dead fans who were skeptical when Mr. Mayer joined Dead & Company, she knew from the very beginning that he would bring something unique to the spinoff band.

“There’s a lot of crusty people who said, ‘Oh, I can’t see John Mayer,” Ms. Marston said. “But if you knew anything about John Mayer prior to joining Dead & Company, then you knew the guy could freaking rip the blues.”

“Sometimes his eyes are rolling back in his head,” added Ms. Marston, who was wearing a black top covered with photos of Mr. Mayer. “It elevates everybody because he’s so into what we’re into — it’s our synchronization with the band.” As she spoke, a man with a fake scarlet begonia tucked into his hat interrupted her to show off a sticker that featured Mr. Mayer’s face flashing a particularly euphoric expression and surrounded by a highly suggestive lyric from the song “The Weight.”

A few Dead & Company fans said they had never noticed Mr. Mayer’s expressions. Kim Holzem, 52, from Three Rivers, Calif., scoffed in disbelief when her husband, Tim, mentioned that he had never registered the guitarist’s faces before.

“Sometimes he looks like he’s in pain, other times he looks like he’s blissed out,” said Ms. Holzem, who saw Dead & Company three times last weekend in San Francisco with her husband and two teenage sons.

Mr. Mayer, she added, “makes some weird-ass faces, but he’s still adorable.”

Skyler McKinley, 31, a bar owner from Denver who was standing not far from the stage at the last show of the tour, said Mr. Mayer’s face was “inescapable” at live performances, in part because it is often “blown up, to skyscraper size” on massive screens. He added that Mr. Mayer had the “sex energy of a rock star” while performing, and compared his facial expressions to the dance moves of Mick Jagger.