Christopher Lambert originally asked for just $50,000 to appear in the pilot, but one of the French investors protested it was too much money and had him written out. This made the German investors threaten to withdraw, so he was put back in. But he'd gotten annoyed by the situation so he refused and as a result he was eventually paid $500,000 for 3 days of work.
The character of "Matthew McCormick," the immortal who works as a federal agent tracking serial killers, was created specifically for actor Eric McCormack. The producers even considered giving him his own Highlander spin-off series, in which McCormick would travel the country where ever there was a series of beheadings and "investigate" the crimes while simultaneously dispensing immortal justice.
Plans for a spin-off series led to a group of sixth-season episodes focusing on a new female Immortal (a "back-door pilot" in industry-speak). None of these characters caught on, and the established character of Amanda (Elizabeth Gracen) became the lead for Highlander: The Raven (1998).
The series was originally planned to follow the film, with Connor MacLeod in the lead role, to be played by Adrian Paul. Paul, however, wanted the freedom to create his own characterization, and to avoid direct comparisons to Christopher Lambert, so the writers agreed to create a new character.
Adrian Paul loved doing flashback scenes where he was given the opportunity to wear period clothing. He always wanted to do an episode set entirely in the past. He did so in the sixth season episode "Unusual Suspects", set in 1929.
The parts of the show supposedly filmed in America were supposed to be set in the city of Seattle. It was filmed in Vancouver, BC, during those episodes which led to the creation of the fictional town of Seacouver.
The season five finale, "Archangel", was originally shot with a cliffhanger ending set 20 years in the future, when MacLeod is finally prepared to defeat Ahriman. The sixth season was planned to happen in the future, but was deemed too dark early on and was finally scrapped when the budget was sliced.
The series directly follows the feature film Highlander (1986), but with a slight twist: according the series, Connor MacLeod was not the last Immortal and did not win the Prize. In fact, Christopher Lambert guest stars in the pilot episode to pass the baton to series lead Adrian Paul.
Werner Stocker had already died of cancer before the end of the first season, so that episode (which opened with his character's murder) edited together existing footage from earlier episodes for the first scenes.
Although the series ended in 1998. Adrian Paul would return as Duncan MacLeod for Highlander: Endgame (2000) with Christopher Lambert playing Connor MacLeod for the last time and connecting the TV series with the films and Highlander: The Source (2007) which is the final installment of the "Highlander" saga.
Joe Dawson (the character ultimately played by Jim Byrnes) was originally going to be named Ian Dawson, and was conceived to be a bookish historian with less world experience than Joe. Actors considered for the role included Michael York and David McCallum.
After Highlander :The Raven was cancelled in 1999, an attempt was made to develop a Highlander spin off series featuring Methos and Joe Dawson. Peter Wingfield who played Methos wanted to do the series when he was told about it. But the proposed series never matirialized.
Despite his character being killed off in the third season episode Star Crossed, Roger Daltrey wanted to return to the series playing a different Immortal character even telling the producers that he would shave his head and use an American accent in order not to confuse fans. But his idea never happened. Instead he was brought back to reprise his role of Hugh Fritzcarin in flashbacks for the remainder of the series.
Over 400 actors auditioned for the role of Duncan Macleod. Adrian Paul was the very first actor to audition for the role. Dutch actor Derek De Lint was eventually cast as the lead. But he dropped out of the series during pre production.
Ron Perlman and John Rhys Davies were considered for the role of Methos. Perlman ended up guest starring in the fifth season episode of the series The Messenger playing an Immortal pretending to be Methos played by Peter Wingfield.
Christopher Lambert was instrumental in getting the series developed. He had heard Peter Davis and William Panzer expressing an intrest in developing a Highlander tv series and arranged a meeting between them and some tv executives from France during the Canne film festival in 1991. After negotiations went through, the series entered development.
When the character of Methos was being created. The idea was that he would have been a one time character. One idea that was considered was to have Methos find Duncan and leave with him a vast collection of his writing detailing his past 5000 years on earth. He would then leave to die as he was tired of living. When the character finally appeared, the plan was to kill him off in the third season finale. But the sudden mass popularity of the character played by Peter Wingfield led to the producers and writers of the show making him a recurring character. Methos returns in films Highlander IV-Endgame & Highlander V-The Source.
In an interview, Adrian Paul stated that when he saw the poster of "Highlander II: The Quickening", he was going to do something with it and eventually was cast as Duncan MacLeod in the television series.
The series was supposed to end after the fifth season. But the producers decided to renew the series for a sixth year so as to look for a new set up for a spin off series that would become Highlander The Raven.
Was originally conceived as a prequel with the events leading up to The Gathering depicted in Highlander (1986). However it was felt that audiences would find the prospect of the heroes ultimate demise too depressing so it was dropped in pre production.
The character of Marcus Constantine played by James Faulkner was intended as a possible replacement for Darius who would have acted as an elder statesman/mentor to Duncan. The producers decided to use Methos in this capacity and Constantine never reappeared. Also the character of Cederwiyn played be Kim Johnston Ulrich was also considered as a replacement before Methos.
In Spain Christopher Lambert's character (who just appeared in the pilot episode) is known as "Conner MacLeod" after that in the Spanish dubbed voice of Highlander (1986) Connor was turned in "Conner", and all later sequels were adapted in the dubbed voice according with it, including the series.
The trivia items below may give away important plot points.
Early in the second season, Alexandra Vandernoot told the producers she didn't want to be on the show anymore because of her pregnancy at the time - which lead the writers to kill her off in "The Darkness".