INTJs Locke (Lost) and Mulder (X-Files) seem to be commonly mistyped as INTPs. Since it’s generally agreed that both are INTs, the best way to get a fix on that 4th letter is to determine whether their dominant mode is Introverted Intuition (main function of INTJs) or Introverted Thinking (main function of INTPs). It appears more than likely that both are INs (e.g., Locke seeking a mystical union with the Island and his vision quests; "Spooky" Mulder’s “weird” theories) rather than ITs (whose main focus is on logic, problem-solving and analysis). Compare Mulder to INTP character, Data, from Star Trek NG, to see the difference between a driven visionary INTJ and a more flexible and logical INTP viewpoint. Mulder and Locke both fit the profile of single-minded INJs in pursuit of their IN visions of Truth, even if at times they also seem like SP action heroes.
I would add that obsessive INTJs like Mulder tend to get lost in their IN world of theories and ideas and become split off from their balancing, organizing auxiliary Extroverted Thinking function. They don’t always appear as textbook Js like most SJs do, hence Mulder’s messy office and closet that would give a typical SJ character like Monica from Friends nightmares.
Introverted Intuition is like the Twilight Zone, while Introverted Thinking is more like a supple, sophisticated computer. That’s why there’s a substantial difference in focus between INTP “architects” and INTJ “visionaries.”