According to Muir’s Facebook page he started work on Rogue One around November 2nd, 2015 at exactly 1:21 pm:
It worth noting that Muir is really talented guy and he has done a lot of work outside Darth Vader. With the rumors Vader would appear in Rogue One it certainly seems a little more promising. Since Rogue One’s story takes place just slightly before A New Hope there’s a chance Muir is simply working on costumes that have that feel and vibe of the work they did back in the late ’70s, so Rogue One feels like a film set before A New Hope.
Personally, I hope he’s making a Darth Vader mask just like the one he made for A New Hope. Who doesn’t want to see Darth Vader back on the big screen being the best space jerk around? Muir also worked on Star Wars: The Force Awakens, presumably designing the Endor Darth Vader helmet that Luke Skywalker burned in the funeral pyre.
Check out Mr. Muir’s filmography here!
His official Web site is here!
Rogue One: A Star Wars Story will be released on December 16th, 2016.