Peter Briggs wrote a version of the script while the project was still at Miramax. Before writing the script, Briggs re-watched all the "Highlander" movies as well as the TV series, then joined the official Highlander message boards (www.highlander-community.com) and researched what fans wanted in a sequel. He kept his identity secret, but was known to be an insider on the project. He soon became one of the most popular and respected members of the board, and remained so even after he was kicked off the project. He revealed his identity well after he was kicked off the project. It was the death of a close friend on the forum, named Eric that sparked Briggs to finally reveal himself.

Russell Mulcahy was attached to direct the film early in development. Mulcahy disagreed with Peter S. Davis and William N. Panzer on the direction of the story, as he wanted it to be a prequel focusing around the first-ever generation of immortals, thousands of years before the original Highlander (1986), with Adrian Paul only appearing as Duncan MacLeod in a framing story. On the other hand, Panzer and Davis wanted to follow up Highlander: Endgame (2000) and have Duncan appear throughout the whole film. Eventually, Mulcahy left the production, feeling that a repeat of the events that resulted in the critical and commercial failure of Highlander II: The Quickening (1991) was about to happen.

Joel Soisson's version of the script featured Duncan MacLeod living in a gas station in the middle of the desert, next to a run-down airplane. Also in the script, MacLeod smoked, wore cowboy boots and a hat, and had a pet monkey named Connor.

David Abramowitz was called in to rework the original script by Stephen Kelvin Watkins after the producers had doubts about whether it would work. Abramowitz did a major overhaul of the script in an effort to bring it more in line with prior Highlander mythos, but the producers later changed their mind and largely re-instated Watkins' original script, keeping only a few of the modifications Abramowitz made. While the initial, leaked cut of the film credited Abramowitz as a co-writer, he was so displeased with the results that he took his name off of the final version.

Adrian Paul had originally turned down reprising his role of Duncan in this film as he was extremely disappointed on the performance of Highlander: Endgame (2000). He also believed that the series was not worth continuing with. The producers then contacted Christopher Lambert about reprising his role of Connor MacLeod in the movie. But Lambert demanded a lot of money and he was dropped. Eventually, the producers were able to convince Paul to reprise his role.

Dimension Pictures originally intended to film this movie in Africa. Several early drafts are explicitly set in Africa. It was finally filmed in Lithuania around Vilnius.

This is the first live-action "Highlander" film not released in theaters. The film premiered on the Sci-Fi Channel in September of 2007 and was released on DVD in February of 2008.

The film was shot over three months (October to December) in 2005, and was planned to be released in 2006.

The final installment of the Highlander series.

The film was intended to be the first of a new trilogy, but these plans were dropped after the poor critical and commercial performance of "The Source."

This is the first live-action "Highlander" film in which the villain's name does not begin with the letter K. In Highlander (1986), Highlander II: The Quickening (1991), Highlander: The Final Dimension (1994), and Highlander: Endgame (2000), the villains were named Kurgan, Katana, Kane, and Kell, respectively. (The animated film Highlander: The Search for Vengeance (2007) also featured a villain whose name does not begin with the letter K, but it was produced after "Highlander: The Source," despite being released first.)

Adrian Paul has since expressed disappointment in this film, stating he had been read the David Abramowitz script which he appreciated and thought they were going to film that script only to find out while shooting that they were instead filming a heavily rewritten script which barely resembled the original draft.

The character of Kate was intended to return in this film. Barbuscia turned down the role, and the character of Anna was created.

An early cut of Highlander: The Source was released in Russia on DVD in February 2007.

First Highlander movie not featuring Christopher Lambert as Connor MacLeod.

The original screenplay was intended to begin a trilogy but was rewritten to be a standalone film. According to Adrian Paul, the original script "wasn't up to par to begin with" and wasn't fixed but instead ruined through rewrites.

Continuity

The Elder says that as the immortals get closer to the Source, they will grow weaker and lose their immortality. However, when Duncan reaches the Source, he suddenly gains super-speed. This is even dumber because the Guardian flat-out states that the Source is filling him with energy.

Duncan talks about the women he has watched grow old and die--including Kate from Highlander: Endgame. Kate was an immortal. She could not have grown old and died.

When Duncan fights the Guardian near the end of the film, his wife Anna is standing in the "Source," at the top of a set of stairs at one end of the pit. After the Guardian stabs Duncan, he is standing between the two of them, with Duncan and Anna at opposite ends of the pit. Watch the close-up of Duncan just before the Guardian begins to spin a tornado by waving his sword; you can clearly see Anna standing at the top of the stairs behind him, even though she's supposed to be behind the Guardian.

In the scene at the monastery, Duncan and Joe breaks through the monastery door with Joe's jeep. A few minutes later, as the Guardian arrives, Reggie goes out to fight him, and the monastery door is unharmed.

Some of the immortals continually say that they are on holy ground (and thus, cannot fight), even though the Guardian blatantly kills one of them on holy ground with to no consequence whatsoever.



Factual errors

If Jupiter and Saturn were closer to the Earth than Earth's moon (as is shown late in the movie), the entire solar system would be destroyed due to gravitational imbalance--let alone Earth being destroyed.



Miscellaneous

How did this world descend into chaos in what couldn't possibly be more than a 5-year time difference between this movie and the previous one? Joe Dawson is the same age as he is in the Highlander TV series, so there is no way takes place a long way into the future.

Why did the security guard not attempt to retrieve the body of the man he just shot over half a dozen times? And why did he not call the police?



Plot holes

Methos and Giovanni clearly know who the Guardian is when they are talking via their computers. However, when they all go to speak to the Elder, they seem to have no idea who he is as the Elder tells the story of his becoming the Guardian of the Source.

The elevator takes Zai Jie to the 20th floor of the Comm Tower, even though he was unconscious and brutally wounded when the elevator door closed behind him.



Character error

The Guardian doesn't run after Joe and Duncan, even though he is easily faster than Joe's pick-up truck.