It might not be what you expect.
In an interview with Fox D.C. 5, as pointed out by Entertainment Weekly, J.J. Abrams explained what surprised him most about Rian Johnson's take on the series with Star Wars: The Last Jedi.
“What I loved about his approach was that he was just subverting all expectations everywhere you looked. Maybe the biggest surprise was not the biggest [for everyone else],” said the director. “Weirdly, for me, the thing that was the most surprising was Phasma dying. That was one of those characters I thought that there was something else [for].”
It was a moment of honesty from Abrams who could have dismissed it to avoid accidentally offending Johnson. He continued:
“Look, no one wants a character to die, and yet, I know that when we had Kylo Ren kill Han Solo, that was done because Harrison always knew there needed to be utility for the character, and he had famously always wanted Han to die and serve that purpose. It felt like this was a way to begin to define Kylo Ren, not just a way to kill a character. So, I can see why Rian chose to do that with some of these characters, but I guess for me the biggest surprise, weirdly, was Phasma dying the way she did.”