Palpatine returns to the big sceen in Star Wars: Rise of Skywalker. But it didn’t only seemed strange to fans of the franchise. As it turns out, Ian McDermid, who played the character, was also surprised by such a plot twist.
The actor, just like the audience, believed that Darth Sidious was dead. Moreover, George Lucas himself told him about this after filming Return of the Jedi.
Dermid said “I thought I was dead! I thought he was dead. Because in Return of the Jedi , I was thrown into galactic hell. Then I asked: “Will he return?” To which Lucas replied: “No, he is dead.” So I just accepted it. But then, of course, I still did not know that I would star in prequels. Therefore, in a sense, he was not dead. We returned at a time when he was still young. I was surprised.
Source: BGR