Fake Brit: John Hillerman, a native Texan, playing a retired British Army Sergeant Major — and playing him so convincingly even British viewers thought he was a genuine Brit.
In "Luther Gillis, File #001", Higgins insists on accompanying Gillis into a cowboy bar. When Gillis says they won't talk to a "limey", Higgins grabs Gillis by the lapels and intones "Who you callin' a limey, boy?" in a broad Texas drawl. He played the whole bar scene this way.
The producers actually planned to reveal Orson Welles as Robin Masters when the series ended. Unfortunately, though, Welles died in 1985 before the storyline could materialize.
Don Bellisario's later show Quantum Leap was going to have a crossover episode, but unfortunately the Magnum series was established as existing within that show's universe, and he preferred to avoid the Celebrity Paradox.
Jack Lord was approached several times to appear on the series, but he never did (in fact, once Hawaii Five-O ended he never acted againnote except for a cameo in M Station: Hawaii, an unsold pilot which he also executive produced).
Written By Cast Member: Roger E. Mosley co-wrote "Missing Melody," one of the few episodes of the series to focus on his character of T.C.
You Look Familiar: Tons, like Gillian Dobb appearing as different characters in "Mad Buck Gibson", "The Elmo Ziller Story", and "Ki'i's Don't Lie" before finally appearing as Agatha Chumley in "Black on White".
Glenn Cannon playing another doctor, Dr. Kessler in season one's "Never Again... Never Again", before appearing as Doc Ibold in season two's "Try to Remember".
Lance LeGault as John W. Newton in "Missing in Action" before appearing as Colonel Buck Greene in "Memories Are Forever". The voice helps.
Paul Burke as Admiral Kitchner in "Memories Are Forever" before appearing as Admiral Hawkes in "Did You See the Sunrise?".
Kathleen Lloyd as Bridget Archer in season three's "Almost Home" before appearing as Carol Baldwin in season four's "Distant Relative"