Darth Vader's presence in Rogue One: A Star Wars Story has been largely downplayed by those in charge of the film. While director Gareth Edwards has discussed the fun and excitement of getting to have Vader on the set of his film, he's also cautioned that he's by no means the primary antagonist. That role belongs to Director Orson Krennic (Ben Mendelsohn), the man in charge of the Death Star project. Lucasfilm president Kathleen Kennedy has likewise described his role as more of a looming threat than a direct one.
"He will be in the movie sparingly. But at a key, strategic moment, he’s going to loom large," Kennedy said of the dark lord of the Sith.
That key moment, most would assume, may revolve around the moment that the Death Star plans themselves are stolen. The entire mission of the film comes from the opening crawl of Star Wars: Episode IV - A New Hope, after all, describing the Rebel Alliance's first victory against the Empire, the acquisition of those plans. If that's the pivotal moment, then it would stand to reason that Vader would come face to face with the band of Rebels led by Jyn Erso (Felicity Jones). We've seen shots of Erso, Captain Cassian Andor (Diego Luna), and the droid K-2SO infiltrating the Empire, after all.
But would Darth Vader need to use his lightsaber in such an encounter? Well, it could go either way. While we know the Death Star laser will be at least tested (though not at full power yet) in the film, we have not seen the familiar red glow of Vader's lightsaber in any of the footage so far released. The lightsaber is as much a defensive weapon as it is an offensive one - perhaps even moreso - even for the Sith. A powerful Sith Lord tends to reach out with the Force first, long before they'd ever bother to ignite their laser sword - when you can choke someone from across the room, or even across space, why bother waiting until you can drive your sword into them, after all?
However, if a group of individuals were to open fire, then that lightsaber may come out to deflect the fire. The other most common situation for it to be ignited is, of course, when facing down another lightsaber wielder - of which we're highly unlikely to see one in Rogue One. The movie is about the non-Force using members of the Rebel Alliance (a "new hope" is yet to join the fight), so someone suddenly lighting up the weapon of the Jedi would be a strange move.
The only reason, storywise, for Vader to use his lightsaber would be to deflect blasterfire. Will that happen? Almost certainly. The shot of that red beam emitting from his hilt is too good to pass up. Perhaps he's underfire while they're making their hasty retreat and ignites for that purpose, but even with limited screentime, it should happen at least once.
The other thing to consider, of course, is that Darth Vader has to come out of any and all screentime he has looking about the strongest and scariest he ever has. This is the Darth Vader who would kill his former master without hesitation, or Force choke a subordinate just because he's mildly irritated. Looking at more recent additions to canon, this is the Vader who a few years earlier slaughtered an entire village and a group of Stormtroopers for the "crime" of seeing the Emperor use his Force abilities (in the novel Lords of the Sith). This is the Vader who can take on entire battalions of starfighters single-handedly (in both that novel and Star Wars Rebels), or who can have an AT-ST dropped on his head, and come out no worse for wear, chucking it aside with the Force and showing that he's an unstoppable and inevitable source of death to any who would challenge him. In other words, if Darth Vader does ignite his lightsaber in Rogue One: A Star Wars Story (or comes into contact with the Rebels at all), it almost certainly means death for one or more members of the Rebel Alliance team in the film.
Rogue One: A Star Wars Story hits theaters December 16, 2016. Darth Vader is once again voiced by James Earl Jones, who returned to the role last season on Star Wars Rebels.
READ MORE: New Rogue One Featurette Highlights Links to A New Hope | Does Jyn Erso Have a Major Star Wars Artifact in Rogue One? | Alan Tudyk Warns New Star Wars Droid K-2SO Could Be A Danger | Before K-2SO: A History of Star Wars' Most Killer Droids| Review: Incredible Star Wars Fan Film Hoshino Explores Nature of the Force with Innovation | It Was Donnie Yen's Idea for his Rogue One Character to Be Blind | Star Wars Rebels: Thrawn Returns in Iron Squadron Preview