One of the few actors to return from the original trilogy, him being cast as the Emperor so young worked out for him when it came to the prequel movies.
During The Phantom Menace, we see him first as Darth Sidious who is working with the Trade Federation to hurt Naboo and the Republic.
Which was just a scam to get him elected as Supreme Chancellor, as he was Senator Palpatine from Naboo.
And he’s convincing enough to get the Jedi to give him emergency powers which makes him mostly a dictator. But the Jedi grow concerned when he spends too much time with Anakin Skywalker.
And he was grooming him to be his Sith apprentice and pushing him to the dark side, little by little.
And he sure played that boy like a fiddle.
Revenge of the Sith shows for the first time that he actually does use a lightsaber and he’s quite skilled at it. Killing several Jedi Masters with ease and being able to defeat Master Yoda.
And his quest to become the one ruler of the Galaxy and to commit genocide against the Jedi is mostly successful.
I was a little surprised that he kept Anakin alive after seeing him like this. And he later (in the original trilogy) came to regret that.