While Darth Vader was part of Rogue One: A Star Wars Story even in the early stages of development, his role actually changed quite a bit as the film’s script went through multiple reworkings. The plan was always to use Vader sparingly and to showcase the character’s merciless nature, but rather being displayed in the brutal corridor slaughter involving a bunch of unlucky Rebel soldiers, an early draft of the script featured the Dark Lord of the Sith executing a member of the Imperial high command who failed him.
“Vader is in the movie as much as he always was,” says Gary Whitta, who wrote a version of the Rogue One script that was later redrafted by Chris Weitz and later Tony Gilroy. “He only had two things in the film. He was on Mustafar and then in the battle at the end.
“The rampage where [Vader] murdered everybody wasn’t me. that go added later,” said Whitta. “I had pictured early on Vader murdering all these Rebel soldiers but I never wrote it into the script. It was an idea that stuck around after I left and they ended up finding a cool way to use it. It’s actually my favorite thing in the film.”
However, Whitta did have a bonus Vader scene at the ended that ultimately got dropped from the storyline.
The original idea was for the Death Star to show up at the Battle of Scarif and perform the same partial blast that destroys the attacking Rebel soldiers along with the Empire’s weapons facility. In this version of the story, both Jyn Erso (Felicity Jones) and the character who later became known as Cassian Andor (Diego Luna) escaped with both the Death Star plans and their lives. There was another survivor too: the villainous Director Orson Krennic (Ben Mendelsohn).http://www.starwars.com/films/rogue-one Source: StarWars.com