Long before the awesome season one finale of Star Wars Rebels, we knew that the second season would begin with appearances by Vader, Ahsoka, and The Emperor. At that time the voices of Vader and Ahsoka were known, but the voice of the Emperor wasn’t.
MakingStarWars has heard that Sam Witwer will be taking over to voice Palpatine in the second season. Tim Curry voiced Palpatine in the final episodes of Star Wars: The Clone Wars after Ian Abercrombie passed away. Sam Witwer is a Star Wars veteran having voiced Darth Maul as well as the Son in the Mortis arc.