When young Agent K first appears, he says "We'll take it from here", the first line K said in Men in Black (1997).
The woman who lives in K's apartment in an alternate timeline, is the same woman in Men in Black II (2002), a waitress, whom Patrick Warburton's character is told to marry and have a bunch of kids.
Young Agent K is wearing the watch that will become the light source for the Grand Central Station locker creatures in Men in Black II (2002). This can be seen clearly when young Agent K and J are having pie before the Mets fans enter.
Shea Stadium, which also appeared in first Men in Black (1997) movie, had to be recreated through CGI for this film, as the stadium was demolished in 2008.
The zip line escape system shown at the Apollo launch pad really did exist. It was installed for the Apollo program and enhanced for the Space Shuttle program. In some pre-launch emergency scenarios, the crew would have ridden steel cages down the zip lines to explosion-proof bunkers. Astronauts practiced using the system as part of their training, but it was never used in an actual emergency.
Frank the Pug does not appear in this film, but his picture is seen in two places: J's apartment, and a carnival poster at Coney Island (seen when J is pulled over).
The white weird looking 'alien' fish which can be seen in the Chinese restaurant's kitchen scene is in fact an actual fish, Psychrolutes marcidus (or blobfish)
Began filming without a completed script, which led to a delay in production, so the screenplay could be rewritten and completed.
Production delays forced Alec Baldwin to drop out of the film. Alec was the original choice to play Chief X, the head of the Men In Black agency in the sixties.
This is Will Smith's first film in 3.5 years, since the release of Seven Pounds (2008) in December 2008. This is the longest he has gone without appearing in a movie since his film career started in 1993.
The prosthetic makeup Jemaine Clement had to apply for his role as Boris the Animal, took four and a half hours.
Many of the aliens in the 1969 MiB headquarters were intentionally made to look like aliens from old black and white alien movies from around the same time.
Although the World Trade Center Twin towers are already standing (but still under construction) in 1969, they cannot be found anywhere in the 1969 parts of the movie, but the Freedom Tower (or One World Trade Center) can be seen under construction in 2012 near the ending of the movie.
According to the trailer Agent J travels from 2012 to 1969, 43 years into the past. He wakes up and sees a young Agent K, played by Josh Brolin. Brolin is only 21 years and 5 months younger than Tommy Lee Jones, who plays the older Agent K.
This is the second threequel Steven Spielberg produced that involves Apollo 11 in its storyline. The first was Transformers: Dark of the Moon (2011) which focused on a Transformer ship discovered by the Apollo 11 astronauts.
Tommy Lee Jones and Josh Brolin, who play Agent K at different ages, previously both appeared in No Country for Old Men (2007). Like that movie, they share no scenes together.
Josh Brolin, one of the three leads, does not appear in the movie until 42 minutes in, which is more than 1/3 of the way through the running time.
BugEyes, the fictional "creator" of a fake blog (source: www.themeninblacksuitsarereal.com) about investigations concerning the Men in Black, made a cameo in the film as an MiB agent in the background.
Screenwriter David Koepp, who was originally involved with Men in Black II (2002) but left to write Spider-Man (2002), signed on permanently for this film.
Gemma Arterton was originally cast as young Agent O, but scheduling conflicts prevented Arterton from taking the role.
Boris was originally named Yaz in the script and through some time of pre-production. It could be possible that the name change was suggested by Rick Baker, who was a huge fan of Boris Karloff.
The number CRM-114 makes two appearances in this movie in the form of text that appears on the outside wall of the Lunar Max prison (seen after Boris breaks out) and the ID for the bunker on the beach at Cape Canaveral. These numbers are a nod to director Stanley Kubrick, who used this number in his movie Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (1964).
When Jay exits The Chrysler Building in 1969, he is facing The Roosevelt Hotel. In 1969, the hotel across the street from The Chrysler Building was the Commodore Hotel. Opening in 1919, it was completely rebuilt, including being completely covered with a glass façade and reopened in 1980, as the Grand Hyatt Hotel. The Roosevelt Hotel is at the corner of Madison Avenue and 45th Street. Opening in 1924, it would have been a better fit for 1969 New York City than the modern glass covered Grand Hyatt.
Like Star Trek: First Contact (1996) and X-Men: Days of Future Past (2014) starring actor Patrick Stewart, the film is a time travel story and is both sequel and prequel as Agent J travels back through time and meets young Agent K many years before Agent J was recruited into the MIB.
In December 2017, Jaden Smith tweeted that he should star in Men in Black 4, because he "needed some answers" about extraterrestrial life.
Actor Keone Young plays Wu, the owner of the Chinese restaurant. He also played a character named Wu in Deadwood (2004).
This is the first film in series in which Will Smith receives top billing and surpasses Tommy Lee Jones in the main credits.
Nicole Scherzinger listened to Led Zeppelin to get her in the mood for the role of Lily Poison.
At 47:20 into the film when J is in the large neuralyzer, the SFX you hear when the machine is shutting down is the same SFX from The 'burbs (1989) when the Klopek's are in their basement and Art, Mark and Ray are spying on them from outside (at around 27:50) - This claim has since been dismissed by the film's sound designer...
When Jay enters the music store, the clerk behind the counter is playing air guitar (and singing the wrong lyrics) to the Black Sabbath song "Children of the Grave" from their 1970 album "Master of Reality".
Chloe Sonnenfeld: Barry Sonnenfeld's daughter appears as the hippie girl who meets 1969 Boris at Coney Island. She also appeared as Elizabeth, the young Rugrats (1990)-loving girl at the post office in Men in Black II (2002).
Rick Baker: special makeup effects artist, appears as the alien with an exposed cranium at Z's funeral.
Rip Torn: as Large-Headed Alien at the funeral of Zed, Torn's own character from earlier installments.
Tony Shalhoub: (at around 38 mins) Manning the 1969 Newsstand. Shalhoub plays Jack Jeebs in previous installments.
Lady Gaga: An alien on a screen in the background of MiB headquarters, with her alias as her alien name.
Judy Murdock: Will Smith's personal makeup artist, appears as a blue-skinned alien at Z's funeral.
Yuri Lowenthal: the voice of Knuckles the graffiti alien in a deleted scene (which is shown in the trailer), had voiced Ben Tennyson in the Ben 10 (2005) cartoons; the Tennyson family is connected to the Plumbers, an organization similar to the Men in Black.