In the trial, Heard referenced Depp's ex, British model Kate Moss, and an unverified rumor that Depp pushed her down the stairs while they were dating in the 1990s. She said that's what prompted her to hit Depp for the first time in their relationship during a fight in 2015.
Heard's sister was present during the argument and said she "put herself in the line of fire" at the top of a staircase to protect her.
"I don't hesitate. I don't wait. I instantly think of Kate Moss and the stairs and I swung at him," Heard added.
"In all of my relationship to date with Johnny I hadn't landed a blow," Heard testified. "And I, for the first time, I hit him, like actually hit him square in the face. He didn't push my sister down the stairs."
Moss has never accused Depp of domestic violence.