Via ScreenRant.com:

The Emperor was almost played by a different actor in Return of the Jedi, and it would have changed Star Wars moving forward. While mentioned in the original film, the Emperor did not appear onscreen until The Empire Strikes Back, where he communicated with Darth Vader via hologram. In the 1980 theatrical cut of the movie, the character was physically portrayed by Marjorie Eaton, and Clive Revill performed the voice. With the Emperor set to have a larger role in the flesh during Return of the Jedi’s third act, the decision was made to cast somebody as the character. As many fans know, the job fell to Ian McDiarmid.

Click below for the full story at Screen Rant