During the run of the X Files, it had a hardcore group of fans, who did things like try to find clues to the mytharc found in the DVD upon which commentary was produced. I can't recall the exact circumstances, but the mytharc was a mystery and the Internet was abuzz with where Carter was going. He then gave a clue within some form of media...I believe it was in CD form. It might have been an early DVD.
As pieces of that mytharc came together, and with lots of interested in science fiction films from not only America, but from diverse places, people looked at the common themes, and unquestionably some things happened on Quatermass that also happened on the X Files.
Looking back at the episodes and in stills, one is struck by the incredible beauty of Gillian Anderson. She never seemed to find her niche in Hollywood, but did find later success in England. She's been featured on numerous productions, and among them The Petal and The Rose as well as the recent serial killer drama The Fall.
Here's a forum post which discusses the hidden track of commentary by Chris Carter.