Such was Chris Carter's standing with the FOX network at the time, that he was given an entire month to shoot the pilot with little or no network interference - almost unheard of indulgences for a brand new show.
Frank, Lara and Peter all get pass-phrases to use for the voice identification of the Millennium Group network. All are references to classic science fiction movies: Frank's phrase "Soylent Green is people" is taken from 'Stanley R. Greens' Soylent Green (1973), while Lara's pass-phrase "Open the pod bay doors please, Hal" refers to Stanley Kubrick2001: A Space Odyssey (1968). Peter's is "My God! It's full of stars!!" from 'Peter Hyams' "2010: The Year We Make Contact" (1984).
The FOX network considered casting William Hurt as a more well known actor in the role of Frank Black but Chris Carter was adamant that the part should go to Lance Henriksen since he wrote the character of Frank Black specifically for Henriksen. When Hurt confirmed that he won't do television, Carter's choice of Henriksen was green-lighted by the studio.
During a 2016 interview on Kumail Nanjiani's podcast "The X-Files Files," "X-Files" writer and "Millennium" showrunner Glen Morgan said that because "Millennium"'s ratings were low, they had the idea to do a "Millennium" episode involving the Peacock family characters from the "X-Files" episode "Home," for which Morgan was the cowriter. The Peacocks were a reclusive, heavily deformed, murderous family that had sustained itself by inbreeding for many generations and that kept its limbless matriarch under a bed. The original airing of "Home" had been controversial; it was the first "X-Files" episode to receive a TV-MA rating, and FOX refused to allow it to be rerun. But since it was also one of the highest-rated and most-talked-about "X-Files" episodes ever, Morgan thought the return of the characters might boost "Millennium"'s ratings. They cleared the idea with Peter Roth (then the head of FOX TV) and Karin Konoval, the actress who played Mrs. Peacock, but then Roth called Morgan back and said that "News Corp, FOX, lobbyists in Washington...somehow got wind and said 'those characters never appear on television again.'"
The title sequence in the first season includes the words "Wait", "Worry" and "Who Cares?". This changed in the second season to "This is Who We Are" and "The Time is Near". Season three combined the two, with "Wait", "Worry" and "The Time is Near".
Frank Black's yellow house is different in the rest of the series from the pilot as the original neighborhood didn't want a film crew camped out there on a semi-permanent basis. The replacement house was the same house as used in the second ever episode of "The X-Files".
In the opening title sequence, and in many promotional materials, the word "Millennium" was spelled with two upper-case M's (MillenniuM) The Roman numeral MM means 2000, the year which marks the turn of the Millennium.
In the season one episode titled "The Judge", the Judge mentions the Biblical story of the demon Legion. The Judge says "When Jesus of Nazareth cast demons from a group of enchanted hogs...". This is inaccurate. First of all, Legion was inside of a crazed man. Jesus cast Legion from the man INTO the hogs. There was nothing enchanted about the hogs. Also, Legion explained to Jesus, "My name is Legion for we are many." This implies that many demons had possessed the man and the strongest of them spoke as if they were all one entity.
Reportedly, the character of Lt. Bob Bletcher was written out of the series (in the episodes "Lamentation" and "Powers, Principalities, Thrones and Dominions") because of an on-set feud between Bill Smitrovich and Lance Henriksen.
Lance Henriksen had always insisted that his character of Frank Black is not psychic, arguing that having to use intelligence or naturally developed skills to solve something is more interesting than having some supernatural power that makes everything easy like a cheat. He always believed that Frank's unusual talent comes from his photographic memory, honed skills of deduction and deep understanding of the dark side of human psyche as well as the vast experience he got from working in the field instead from an office or a desk.
According to Lance Henriksen, there were two main reasons for the increasing inconsistencies in the show's main story arc throughout the seasons. For one, the show's creator Chris Carter is notorious for refusing to answer all of the questions his stories present. The other reason is that the show would regularly get new writers who all wanted to add their own ideas to the show and would ignore or abandon the previously set threads of the main story arc, in the process.
Having no real formal education, Henriksen saw Frank Black as a character much smarter than him and studied various books on psychology, the FBI and profiling for the role trying to capture the essence of such a well-versed character. He's still amazed that he pulled it off.
According to Lance Henriksen, the fact that he was nominated for Golden Globes for his role of Frank Black each year the show was on but never actually ended up getting the award probably means that he was nominated in the first place thanks to the writers who wrote a good character for him and not his acting.
The series' significant shifts in tone over its three seasons were mainly due to the fact that creator Chris Carter ran the show for its first season, but then took a backseat as he was busy making The X Files (1998). Executive producers Glen Morgan and James Wong took over as showrunners, moving the show away from its serial killer theme to include more occult and supernatural elements for the second season, in the hope of attracting more viewers. Carter did not like this new direction, so he took back creative control for the third season, while Morgan and Wong left the series to pursue projects of their own. Carter was hoping to boost the show's ratings by putting less emphasis on the supernatural and returning to a detective-themed show, but could not prevent the show's cancellation after the third season.