Trivia: Music composer John Williams has a cameo as a bartender on the planet Kijimi.
Trivia: Anthony Daniels once again appears as C-3PO, making him the only actor to be in all Star Wars films. Similarly, C-3PO and R2-D2 are the only characters to appear in all 9 Skywalker saga films. Sadly R2-D2's original actor, Kenny Baker, passed away in 2016.
Trivia: C-3PO's final line of dialogue, "Did you hear that?" is also the first line he speaks in the entire series, in Episode IV, "A New Hope."
Trivia: The opening crawl makes reference to Palpatine announcing his return from the dead through a broadcast. The film itself doesn't reveal what was said in the broadcast, but it was revealed in a Star Wars Fortnite event. The exact broadcast is as follows: "At last the work of generations is complete. The great error is corrected. The day of victory is at hand. The day of revenge. The day of the Sith."
Trivia: Boolio, the Ovissian who informs Finn and Poe of the First Order spy, is voiced by Mark Hamill (Luke Skywalker).
Trivia: Director JJ Abrams has a vocal cameo as the droid D-0.
Trivia: Sally Guinness, granddaughter of Alec Guinness (Obi-Wan Kenobi), has a cameo as one of the First Order officers during the scene where Kylo Ren addresses a small council.
Trivia: Rey's name translates to "King" in Spanish. Though there's no evidence to suggest this was ever meant to be a clue to her heritage, you can still find internet posts from as far back as 2015 in which fans speculate that Rey could be a descendant of the Emperor based entirely on this sole bit of information.
Chosen answer: According to the novelization, Palpatine sensed Vader's internal conflict and created a clone as a backup in the event that Vader betrayed him. When Vader threw him down the shaft, Palpatine transferred his consciousness into the clone's body.