The multitude of new faces in Rogue One: A Star Wars Story have received most of the attention in the marketing campaign thus far, but there is one legacy character that fans cannot wait to see up on the big screen again. We speak of famed villain Darth Vader, who has an unspecified role in the spinoff. The Sith Lord is expected to have a minor, yet impactful, presence and has been shown briefly in a few of the Rogue One trailers. His inclusion is a key selling point for casual audiences and die-hards alike.
Since Vader has barely factored into the promotional materials, many have been wondering how exactly he fits into the story. It's safe to assume that Vader is not the main antagonist, as Director Orson Krennic seems to have that aspect covered thanks to his relationship with Galen Erso. Word is that Krennic is attempting to move up the Imperial hierarchy by delivering his Emperor a fully operational battle station, and this is where he comes head-to-head with Vader.
Star Wars News Net got hold of a description for a scene depicting a conversation Krennic has with Vader. Apparently, the two have a "heated exchange" shortly before the Death Star fires on Jedha, the spiritual moon that has a strong connection to the Force (something seen in the international trailer). Per the source, Vader wants to "discuss the reason why they shouldn't force their hand until the right time." On this debate, Vader probably feels it's best for the Empire to wait to use the super weapon (he was never all that proud of the "technological terror"), while Krennic is eager to demonstrate its power and please Palpatine.
No doubt, many fans are hoping that there's more Vader in Rogue One than just that one sequence. From the trailers, it's safe to assume that he will appear in a couple of scenes, but time will tell. Viewers would like to see Vader get involved with the action at some point, so ideally director Gareth Edwards gave the villain one jaw-dropping set piece. The good news is that the advertising materials have done such a good job of getting moviegoers invested in all the new content that minimal Vader is an expectation and won't be a detriment to the final product. Still, this is likely the last time Vader will be in a film, so it'd be great if he goes out with a bang.