The main characters are from the same home states as the actor playing them: Reese, like Jim Caviezel, is from Washington state; Finch, like Michael Emerson, grew up in Iowa, both Root and Amy Acker are from Texas.
Shaw is of Persian descent, like Sarah Shahi, and has both a Persian and westernized first name; not to mention both has the initials S.S. Her frequent use of football metaphors ("I know this went sideways, Finch, but you can't bench me for every mistake." "You go through identities like they're Dixie Cups, but I'm one and done?") are a reference to Sarah Shahi's former career as a Dallas Cowboys cheerleader.
Michael Emerson saying "I've been watching you for a long time, John" sounds awfully familiar.
So does the sound of a receipt being printed in a taxi cab that just so happens to be driven by Michael's character. LOST's sound effect for the "smoke monster" was derived from this sound.
Reese: "Where did you come from?" Finch: "I breached the space/time continuum. Not really." (Yes, really).
In "Witness," a student of the teacher John is protecting is reading The Count of Monte Cristo as a homework assignment. Jim Caviezel played the title count in the 2002 version of the movie.
Doubles as a Whole Plot Reference. The Count of Monte Cristo is about a man who devotes his life to revenge, heedless of the cost to others—which is a pretty fair description of that episode's POI, Carl Elias.
At one point Detective Carter tells Reese that he can't keep playing God.
May be a coincidence but Root was initially believed to be a girl who went missing from a library years earlier. Though it turned out to not be her, that particular backstory is not unlike another certain character portrayed by Amy Acker.
Root also seems to have a thing for tasers, not unlike Fred.
Inverted in Liberty. The POI, Jack Salazar, is a member of the Navy who comes into conflict with Marines throughout the whole episode. He is played by Ray Valentin, who played Gabe Garza, a U.S. Marine, in Generation Kill.
In Reasonable Doubt, Shaw has to go undercover to a book club. She introduces herself as "Mindy's friend, Carmen."
Saul Rubinek plays a genius named Arthur who has ties to the U.S. government working on a secret project? Hmmm...
The Cast Showoff: Kevin Chapman is an amateur boxer, so Fusco displays these skills on occasion.
The Danza: John Greer is played by John Nolan, though it's a moot point in that everyone uses Last Name Basis with him and the fact it's been confirmed to be an alias.
Executive Meddling: A positive case — in the pilot's original script, Diane Hanson's exposure is only seen remotely. A scene was added in which Reese substitutes his recording of her talking to her dirty cops for evidence she's presentingin court.
Person of Interest is a virtual refuge for actors who’ve been in crime/urban dramas in recent years. Just to name a few;
Detective Lionel Fusco, Reese’s (not so) Dirty Cop lackey is Freddie Cork. He was also the prison warden in 24 season 3. In one episode, he got threatened by a drug dealer who was played by the same actor as Hector Salazar also on 24.
Bill Rawls has become a senator of dubious ethical standards.
Invisible Advertising: The amount of advertising the show gets is woefully out of proportion with how high its ratings are; it's one of the highest rated shows in the country, let alone on CBS, and yet it's rarely advertised except on the night that it airs and it's often absent from the group CBS network promotions.
The opposite was true when the show was first introduced. CBS promotions went into overdrive during the summer of 2011 to hype the show up, with POI ads outnumbering those for fellow rookie Unforgettable by about two to one. Ironically, it had the opposite effect in the ratings - the premiere dropped nearly forty percent in 18-49 from the second half of a special hourlong episode of The Big Bang Theory (5.1 to 3.1), and was outrated by all but two of CBS' other Monday-Thursday dramas. Given the pedigree and heavy marketing, this was considered a disappointing start. The only upside is that the show stayed in the general vicinity of this number for its entire run on Thursdays, while several other CBS shows fell off.
Sadly, the show was moved to Tuesdays at 10pm starting in season 3, a budding curse slot for the network which has killed almost every show placed there since Judging Amy went off the air, and is down year-to-year. Facing Sons of Anarchy, one of cable's most popular series, which has grown year-to-year for three straight seasons, isn't helping. Neither is the continuation of this trope in the new slot.
Mean Character, Nice Actor: True, this can be said of anyone who isn't a serial killer but plays one, but there are some notables:
Jim Caviezel, playing an Anti-Hero killing machine is a devout Catholic with two young children.
Enrico Colantoni playing affable but ruthless mob boss Elias is a spokesperson for the Tema Conter Memorial Trust, which helps first responders and such with post-traumatic stress disorder.
Robert John Burke, playing the memorable and vile Patrick Simmons spent several days at Ground Zero after the September 11 attacks assisting with the search and rescue. He has since become a New York State certified firefighter. He also recently traveled to New Orleans to assist in the re-building of New Orleans fire houses.
Screwed by the Network: Due to falling ratings, CBS renewed the show for a shortened fifth season but gave no indication as to whether or not it would be the final season. Aside from press releases saying that production had begun and some noncommittal interviews with the cast and showrunners, there was no promotion nor any indication of when new episodes would air. It took months before the announcement that the fifth season would be broadcast at the very end of the 2015-2016 TV season, long after production wrapped, and new footage only started being released in April, 2016.
The Cast Showoff: One of Kevin Chapman's hobbies is boxing and in several episodes, he's put his skills to use.
Throw It In: Reese kissing Carter at the end of "The Crossing" was improvised by Jim Caviezel.
Trolling Creator: It is a truth universally acknowledged that the POI writing team gets a sadistic pleasure out of TROLLING THE FANDOM.
What Might Have Been: The original script for the Pilot episode revealed all of John and Jessica's backstory in that episode, and would have also included a scene where Finch provides Reese with an apartment to live in. The John/Jessica backstory was cut down and then spread out over the course of the first season, and the apartment became a birthday gift in "Many Happy Returns".
Carter's pursuit of Reese was originally intended to continue over the course of the whole first season, but she ended up joining the team mid-way through due to pressure from CBS.note Mileage Varies on whether or not it was a good move for her to join so soon.
The showrunners have hinted – perhaps jokingly, see Trolling Creator – that they had planned to kill Reese before Sarah Shahi's pregnancy changed things.
Why Fandom Can't Have Nice Things: The reaction to the ending of "The Crossing" has forced the show runners and Taraji P. Henson to go into something akin to damage control mode.
Taraji P. Henson: "That's why people are so distraught today. And that's what art is supposed to do: It's supposed to make you feel. You're supposed to be mad and want to throw a glass and yell, "Goddammit, I'm not going to watch this show again." We did our job."
Written-In Infirmity: Shaw's exit from the show was due to Sarah Shahi having twins and planning to take two years off the show - the reduction of Season 5 to 13 episodes led to her reducing that leave and she will will appear in eight or nine of them.
Jim Caviezel is ambidextrous in real life.
Reese uses the Center-Axis Relock shooting style, first popularized in the video game Splinter Cell Conviction.
While on location in New York City, Jim Caviezel saw some punk beating his girlfriend and "intervened", an incident which has caused some fans to come to the conclusion that he is John Reese.
The (stolen) black Porsche that Reese drives in "Bury the Lede" actually belongs to Jim Caviezel.
Which in a stroke of FridgeHumor, incidentally implies that John Reese stole Jim Caviezel's car.