Watts stars as the late princess in Diana, a movie that centres on the final two years of her life. It also explores her affair with heart surgeon Dr Hasnat Khan, played by Lost's Naveen Andrews.
Speaking to InStyle UK, Watts joked that she may have to escape the UK once the film is released.
"I might go into hiding," she said. "I might have to leave the country."
Watts explained that she nearly refused the role on several occasions, saying: "More than twice I thought about not doing this.
"I said no before I said yes just because taking on that challenge is a massive one. There were weeks of deciding, and then I thought, 'God, I don't like the idea of anyone else playing that role'."
She added that a backlash from viewers is expected,