Quincy Jones famously attributed Michael's enormous success to his "ass power":
At one point I asked Q what separated the great stars from the near greats he'd worked with. "Ass power" was his reply. To illustrate his point, Q compared Michael Jackson to another well-known vocalist he'd produced. The other singer, an artist with an immense voice and an insatiable appetite for cocaine, would come to the studio, maybe lay down a scratch vocal, and then wander off for hours. Jackson, in contrast, would come to the studio, record a strong lead vocal, work the stacked harmonies that distinguished his work, and practice where to place those ad-libs that were his trademark.
"His ass power," Q said, "would keep him in the studio until he felt he'd accomplished something that day. That ability to focus, to stay in that chair in the studio, listening to playback and then going back in to record some more -- that's what separates the good from the great."
(From City Kid: A Writer's Memoir of Ghetto Life and Post-Soul Success by Nelson George)