As for this other classic character, that's slightly trickier to pin down. Since Rob Brydon already has Lestrade covered, that leaves two primary options left. The most likely is Mycroft Holmes, Sherlock's older brother who is smarter than the detective, but isn't nearly as ambitious, preferring to sit on the side and work out the details rather than do any legwork. (That could be a good fit for Ralph Fiennes.) The other possibility that comes to mind is Sebastian Moran, Moriarty's second-in-command. There's also the chance that Laure or Fiennes are playing another one of the recurring inspectors, like Gregson or Bradstreet, but with Lestrade already involved, another inspector seems unnecessary. We'll just have and see what's in store for Fiennes and Laurie in the Holmesian comedy.