David Duchovny

He never really appeared in "Millennium", there was just an episode called Jose Chung's Doomsday Defense (Jose Chung is an X-Files character) and it spoofs Scientology. There's a scene where Selphosophy (the Scientology spoof) is said to have positively changed an actor's fate from being a goofy B-movie actor to becoming a leading man. The actor's name is Bobby Wingood and the photos used on the posters are of David Duchovny. The character of Mulder does exist within the "Millennium" universe, though. You can see Mulder & Scully in Season 1's "Lamentation", although it isn't actually Duchovny and Anderson playing them. It's their doubles but you can't tell because they're in the background. Jose Chung's appearance in the Season 2 episode was further proof, and Frank Black's appearance in "The X-Files" seals the deal. Of course, that means that there's an actor that looks just like Mulder running around, but this is a common phenomenon in the Ten Thirteen shows. If they liked an actor they'd recast him in a different part. Nicholas Lea played a one-off character in "GenderBender" and he was asked to return the following year in the role of Alex Krycek.