Terry O'Quinn's biggest claim to fame is playing John Locke on Lost, but he also played four different roles within the universe of The X-Files. Given The X-Files' long run on TV and the largely standalone nature of its episodes, it's not really surprising that certain actors ended up appearing in multiple roles. Still, with four characters all residing in X-Files creator Chris Carter's world, O'Quinn does hold the record in that regard. Yet, O'Quinn works so often that his X-Files characters are just a few of the over 100 he's played.
Prior to stealing the show regularly on Lost as the island's resident philosopher and adventurer, O'Quinn was probably best known for playing the titular role in 1987 horror cult classic The Stepfather. O'Quinn exuded both menace and unpredictability as Jerry Blake, a man who yearns for the perfect family. The problem is that his attempts to create one invariably lead to things falling apart, and murder being next on his to-do list.
While it's certainly within the realm of possibility that Terry O'Quinn's various characters within the X-Files universe are somehow related, there's been no indication given to suggest that's the case. It would appear instead that X-Files creator Chris Carter is just a big fan of O'Quinn, and enjoys working with him, as evidenced by Carter making O'Quinn a regular fixture on Carter's next 1990s show Millennium. Here's every X-Files universe character played by O'Quinn, what they did, and what happened to them.
Lt. Brian Tillman in "Aubrey"
O'Quinn's first appearance on The X-Files came as Lt. Brian Tillman in "Aubrey," season 2, episode 12. It saw a detective receive a psychic vision revealing the location of a corpse. However, O'Quinn's Lt. Tillman isn't the cop receiving the psychic visions of murder, but is instead her lover. She ends up being the killer herself, but thankfully for him, Tillman survives the episode.
FBI Special Agent Darius Michaud in X-Files: Fight the Future
O'Quinn next popped up in the opening scenes of 1998's X-Files: Fight the Future movie, playing a high-ranking FBI agent named Darius Michaud. Michaud was involved in a Syndicate attempt to cover up the fact that several people had been infected by the X-Files' alien Black Oil virus. He purportedly accompanied Mulder and Scully into an office building as part of the hunt for a bomb, but upon finding the explosive, Michaud made no attempt to disarm it, instead dying in its blast.
Shadow Man in "Trust No 1"
Fast-forwarding to X-Files season 9, episode 6, "Trust No 1," O'Quinn appeared as the mysterious "Shadow Man." He claimed to be an NSA whistleblower with information for Mulder and Scully, but was in fact a super soldier looking to kill them. Scully was cornered, but thankfully super soldiers proved vulnerable to a substance in a nearby quarry called magnetite. This would mark O'Quinn's second X-Files death.
Peter Watts on Millennium
O'Quinn's fourth X-Files universe character was actually born on a different show, Millennium. O'Quinn played the supporting role of Peter Watts on Millennium, which crossed over with X-Files. A member of the titular Millennium Group of former law enforcement personnel, Peter was a close friend and colleague to Lance Henriksen's Frank Black during the first two seasons, then became an antagonist in the third. Interestingly, while Peter Watts canonically exists within the X-Files universe, he never actually appeared on the show itself, as Peter was left out of the crossover episode.