Nicholas Lea (Alex Krycek) originally played a character called Michel in the first season episode called "Gender Bender" in which he is seduced by a religious sect member who can change sexes!
He must've made some friends because just 7 episodes later Nicholas Lea was in the episode called "Sleepless" as the enigmas antagonist Krycek.
This bothered me for so long because I wasn't 100% sure until now.
Thought I'd share.
BONUS FUN FACT
Chris Owens plays the young C.S.M. in the episode "Musings of a Cigarette Smoking Man" (Season 4) and later plays the SON of cigarette smoking man in episodes starting in season 5. This confused me for a bit.
BONUS BONUS FUN FACT!!!
Chris Owens also plays Mutato in my possibly favourite episode "The Post-Modern Prometheus" :)
Please feel free to share some x-files trivia here! I'm always interested to hear more about one of my favourite shows.