The famous echoing chord from the theme music was a fluke. Composer Mark Snow accidentally rested his elbow on his keyboard with the echo function on, and he liked the resulting sound so much, he wrote the theme around it.
During the opening titles, Mulder and Scully's FBI ID badges read "Federal Bureau of Justice, United States Department of Investigation." The alteration was necessary as making a fake FBI badge, even for fictional purposes, is illegal.
When casting the role of the Cigarette Smoking Man, the producers had no idea the character would turn into a major role in the series. When the character became more prominent, they worried that William B. Davis, who had only been cast as an extra, would not be able to carry the part. As it turned out, Davis, who is one of the most respected acting teachers in Canada, impressed series creator Chris Carter so much that he made the Cigarette Smoking Man the main villain of the series.
Props from Mulder's X-Files office are currently preserved and on display at the Hollywood Entertainment Museum in Los Angeles. According to the museum, the famous I Want to Believe UFO poster from the office continually had to be replaced as copies kept disappearing from the set. The poster on display at the museum is reportedly one of the last available copies of the original set-used posters.
Chris Carter wanted the show to end after the fifth season, when his original contract, as well as those of Gillian Anderson and David Duchovny expired. Carter wanted to continue the story in a series of feature films, starting with The X Files (1998). The Fox Network, however, found demand for the show too high to cancel the series, so they instead mandated the film to fit in between series five and six of the show. Carter found writing seasons five and six as well as the film very difficult, as he had to craft a screenplay that would not require the audience to have seen the show to understand it, and episodes of the series that would tie in to the film, but not rely directly on the film's plot to make sense.
Lucy Lawless' character Shannon McMahon was meant to become a major recurring character in the show's final season. A high-risk pregnancy, however, forced Lawless to leave the show after only two appearances.
In one episode, Scully tells Mulder that she thinks Téa Leoni has a crush on him, to which Mulder responds "How could Téa Leoni have a crush on me?" Téa Leoni was David Duchovny's wife in real-life at the time.
The character Leyla Harrison was named after a writer of The X-Files Internet fan-fiction who died of cancer in February 2001. The fictional Leyla Harrison was a fan of Mulder's and Scully's, having spent her time in the F.B.I poring over their expense reports (in much the same way that Internet fans of the show examined the minute nuances of every episode), so the tribute was very appropriate.
The series' science consultant, Anne Simon, a virologist at the University of Massachusetts, wrote a non-fiction book in 1999 titled "The Real Science Behind the X-Files: Microbes, Meteorites and Mutants".
Gillian Anderson has stated that she based her approach to the role of Dana Skully on Jodie Foster's performance as Clarice Starling in "The Silence Of The Lambs". Ironically, a contractual obligation to "The X-files" prohibiting her from playing an FBI agent in any other role would prevent her from being offered the part of Clarice Starling in "Hannibal".
The series does not show episode titles on-screen. While this was not unusual even in 1993 (though it is more commonplace in 2004 when virtually no American dramatic TV series display episode titles), what is unusual is that many fans learned episode titles as well as advance plot information via the Internet. The X-Files (1993) was one of the first TV series to be so promoted.
William B. Davis, who plays the Cigarette Smoking Man, is a former smoker who had quit by the time he was cast in this series. At first, the cigarettes he smoked in this role were real, until he became aware that he was having cravings for tobacco when not filming. At his request, the cigarettes used from then on were herbal.
The first five seasons of the show were filmed in Vancouver, British Columbia, Canada. The show often cast local Canadian actors in guest and secondary roles, often reusing the same actors in different parts in multiple episodes. The most prominent example of this was actor Nicholas Lea, who appeared in a guest role in season one before being cast in the unconnected, more prominent role of Alex Krycek starting in season two, but there are many other examples of local actors who reoccurred in multiple roles over the course of several different episodes. These actors include: J.B. Bivens, Lorena Gale, Gillian Barber, Forbes Angus, Larry Musser, Harrison Coe, Hrothgar Mathews, P. Lynn Johnson, and Doug Abrahams (among many others). Each of these actors appeared on three or more X-Files episodes, each time as a different character (Barber played three characters in four episodes).
Chris Carter lists All the President's Men (1976) as one of his inspirations for the series. There are numerous references to the film, including the shadowy informer Deep Throat, meetings in underground car lots, and hints at conspiracies which stretch all the way to the F.B.I.
During the last season of the show, the opening credits included a shot of a list of "FBI Contacts, Witnesses, and Contributors." The names on the list were actually the screen names of posters on the official "X-Files" message board and changed with each new episode that season. Other names on the list were anagrams of characters on the show.
Chris Carter has cited Moonlighting (1985) as an influence on this show, specifically the relationship between Mulder and Scully. Carter also has said that the show is an example of how not to further the story, as the sexual tension between the two leads should never be resolved.
The recurring character Cigarette Smoking Man was never given a name until late in the series. He is referred to as Cancerman in some early episodes. The show's fans took to referring to him as simply CSM.
Robert Patrick, who plays Special Agent John Doggett, is the brother of Richard Patrick, the lead singer of Filter. Filter's music has appeared on the TV soundtrack, "Songs in the Key of X", and the The X Files (1998) movie soundtrack.
Fox Mulder's sister Samantha was abducted while she and Fox were watching an episode of The Magician (1973). When Patty Hearst (Patricia Hearst) was kidnapped by the SLA, she was watching "The Magician" with her boyfriend Steven Weed.
Recurring use of the numbers: 1013 - Ten Thirteen is the name of Chris Carter's production company; his birthday is 13 October 1956. The number 1121 also appears often; it is creator Chris Carter's wife, Dori's, birthday.
In the UK Twentieth Century Fox Home Entertainment released new "Mytharc" multi-part episodes as edited together TV movies on VHS in the mid 1990s. However, these tapes (which usually took the title of just one of the episodes comprising them but were sometimes completely re-named) were extremely expensive and released as numbered "Files" in the wrong order. For example "File 3: Abduction" ("Duane Barry"/"Ascension"/"One Beath") confusingly came after "File 1: The Unopened File" ("Anasazi"/"The Blessing Way"/"Paper Clip"). Although by the third season most mythology episodes were multi- part stories, important plot information relating to the conspiracy could still sometimes only be found in the unavailable standalone episodes to further confuse viewers of these releases. Also, "Colony"/"Endgame" and "Piper Maru"/"Apocrapha" were seemingly forgotten to be included in the series until long after many later episodes.
Several actors & actresses who co-starred or guest starred would eventually go on to prominent co-starring or recurring roles in Stargate: SG-1 and/or its spin-off's, including Amanda Tapping (Col. Carter), Don S. Davis (General Hammond), Mitch Pileggi (Colonel Caldwell), Teryl Rothery (Dr. Janet Frasier), Colin Cunningham (Major Paul Davis), Megan Leitch (Captain Ke'ra), Steven Williams (General Vidrine), Bill Dow (Dr. Bill Lee), Elizabeth Rosen (Lt. Jennifer Hailey), and Jamil Walker Smith (Master Sgt. Ronald Greer)
The trivia items below may give away important plot points.
At the conclusion of season seven, David Duchovny and Gillian Anderson had yet to be signed for additional seasons, and most of the cast and crew assumed the show would end. Chris Carter wrote the episode "Requiem" to function as a series finale, but in the eleventh hour Fox decided to renew the show. Only Anderson agreed to return to the show full time, and as such, Carter created the characters of John Doggett (Robert Patrick) and Monica Reyes (Annabeth Gish) to compensate for Duchovny's absence. Carter also retooled the series mythology to focus on government Super Soldiers, and to eliminate plot threads regarding the alien invasion conspiracy with the hopes that it would revive interest in the show.
Scully's abduction during the second season was originally planned as a way of explaining Gillian Anderson's absence due to pregnancy. Later, it became a vital plot point to the ongoing series mythology.