After Mulder's been shot and visited by the Lone Gunmen at the hospital, Mulder and Byers swap clothes and Frohike and Langly smuggler Mulder-as-Byers out of the hospital. Look closely at the side of "Mulder's" face as the trio are heading out the hospital doorway -- there's a beard there. It's actually Bruce Harwood, not David Duchovny, in that shot. (This was confirmed in a conversation with Dean Haglund, BTW.)