The writer Bob Colacello met Ammann in the 1970s while working at Interview with Warhol. “[Ammann] was the first person I knew to have Belgian slippers—the first to put soles on them so he could wear them outside,” Colacello recalls. “He was always smiling. Five minutes in his presence, and I was relaxed and cool and happy.”

Ammann easily persuaded many to pay high prices for artists he represented in Europe— Twombly and Marden among them. “If you were discovered by Thomas,” Colacello says, citing Ross Bleckner and Eric Fischl, “that was a huge boost. People wanted to do what Thomas was doing. He bought Clementes, so others did too.”

Such dealers could affect artists as much as they did the public. According to the New York painter Donald Baechler, the late Neapolitan gallerist Lucio Amelio was simply magical. “He had a stentorian voice and spoke many languages,” Baechler says. “He acted in Lina Wertmüller films and wanted people to believe he was a CIA agent. He’d fly you to Paris on the Concorde and throw lavish dinners for 40 and then not pay you for a painting. Then you realized that the painting had paid for the trip.” But it was well worth the missed check: Amelio’s high profile in the ’80s helped young American artists like Baechler, Keith Haring and George Condo replicate their domestic success in Europe.

Today’s primary-market brokers in contemporary art tend to be of the same generation as their artists, at least when starting out. The aristocratic Jay Jopling was as young as Hirst, the Chapman brothers and Gary Hume when he presented their early works in 1993 at Jay Jopling/White Cube, the immensely profitable London gallery that began as a project space. With his upper-crust accent and ability to hold the floor, Jopling is a commanding personality. “Jay has power, and power itself can be charismatic,” says Hume. “But look at what he was like before, and you see he always had it. He built White Cube from nothing, and you can’t do that without some sort of engagement.”

There is no one way to decode charisma, much less distinguish it from the glow that comes from success. “I think it’s chemistry and attitude,” says Lucy Mitchell-Innes, a British-born dealer who began her career in New York at Sotheby’s in 1983, when Schnabel’s first plate painting came up for sale. “Julian called me,” she says, describing a scenario that would be unthinkable now, “and told me we couldn’t put that painting in an auction. ‘I’ll swap you,’ he said. ‘I’ll show you another one just as good, and then I’ll buy the first and you can still have a great painting.’ Well, he did keep that painting, and we sold the other for a record price. He made that happen by sheer personality.” On the other hand, she says, Chris Martin, the Brooklyn-based abstract painter, “has a following like a religious sect, but that’s because of his work. If an artist’s work doesn’t hold up, charisma won’t help, even if it does raise the tide of public perception.”

Obviously, the art world is full of competing personalities and compelling eccentrics whose public images make their work more tantalizing. Or perhaps it’s the other way around. Take Jean-Michel Basquiat, whose aura derived in part from his distinctive dreadlocks and an engaging smile. Yet he was often brusque and confrontational. “There was no separation between what went into his paintings and what you connected with in his persona,” says the dealer Jeffrey Deitch, a former consultant who has played a significant role in the careers of many artists, including Koons and Cecily Brown. With his round glasses and crisp suits, Deitch is no slouch at calling attention to himself. But he puts Basquiat in another category altogether. “A conversation with Jean was not a passive experience. He really did embody his artistic vision. ‘OK,’ he would say, ‘let’s lock horns.’ It was electric to be in his presence.”

The artist Matthew Barney, whom many regard as a visionary, can go into a gallery unnoticed, yet he exercises a fascination on art world denizens. It probably helps that he married a rock star, Björk, and has retained the matinee-idol looks that made him a successful J. Crew model while in college and the star of his own video and film productions. In person, though, he is anything but ostentatious and seems driven by his work.