Series Producer Judd Apatow told Vanity Fair in 2012 that whenever he sees an opportunity to use anyone from the show, he does. It is his way of refusing to accept that the show was cancelled, and that all of his subsequent movies are the continuous adventures of those characters.
The creators were determined not to end each show with a typical "happy ending". One notable exception is the pilot episode, which the creators purposely wrote as a self-contained story, in case the show was never picked up for production.
About halfway through the season, it was evident that the show was in danger of being cancelled. The rest of the episodes were put on hold, while the creators wrote and filmed the final episode (which was written in a way as to serve as both a series finale, if the show was cancelled, and a season finale, if the show would've gotten picked up for a second season). Several episodes later, it was cancelled.
Nearly everything that happens to the characters happened to Producer Paul Feig, or other members of the crew. For example, the blue suit that Sam Weir wears in "Looks and Books", the showering in the locker room storyline in "I'm With the Band".
The pilot was filmed in an actual school, and after the show was picked up, school sets were built that matched the original school almost exactly, and the rest of the series was filmed on these sets. The hallway set was actually just one "T" section of hallway, and it was constantly redressed to make it appear as if more of the school was being shown. The extras walking down the hall would pile up at the end of the hallway after every shot, as there was literally nowhere for them to go.
During the opening credits, you can see James Franco and Jason Segal vigorously rubbing their eyes, to give themselves a stoner look. You can also see Seth Rogen smiling before his class picture, which he takes with a grumpy look. Allowing the viewer to see this was intentional, as it was a common trend in the 80s to harmlessly sabotage one's yearbook photo.
Linda Cardellini (Lindsey) and Jason Segel (Nick) dated off-screen for five years. They also dated in the show. They had previously co-starred in Dead Man on Campus (1998). Segel stated in an interview with the Los Angeles Times in 2008, that Cardellini was a great girlfriend, and the title character in Forgetting Sarah Marshall (2008) was not based on her, it was more an amalgamation of a lot of different relationships and break-ups he had.
Where are they now? Linda Cardellini recently starred in Avengers: Age of Ultron (2015). James Franco went on to become a huge movie star, even receiving an Academy award nomination for his work in 127 Hours (2010). Seth Rogen is now a big comedy man, and recently starred in Neighbors (2014) and Neighbors 2: Sorority Uprising (2016). Jason Segel went on to star in the smash hit How I Met Your Mother (2005), as well as comedy movies such as I Love You, Man (2009). Martin Starr is now probably mostly known for his role as Gilfoyle in the HBO show Silicon Valley (2014). John Francis Daley starred in Bones (2005), and co-wrote Horrible Bosses (2011) and Horrible Bosses 2 (2014). Samm Levine mostly has guest appearances in television shows, but also starred in Inglourious Basterds (2009). Busy Phillips starred in Cougar Town (2009).
While the show took place in Michigan, it was shot in California. Outside scenes proved to be difficult to shoot, because no palm trees were allowed to be seen in the shot. If you look closely, you may see at least one.
James Franco, John Francis Daley, and Martin Starr are all linked to the Spider-Man franchise. Franco plays Harry Osborn in the Raimi trilogy (2002-2007). Daley co-wrote the screenplay for Spider-Man: Homecoming (2017), and Starr plays a role in that same film.
Busy Phillips was on the fence about playing Kim Kelly until Linda Cardelini convinced her, they both were fellow classmates at Loyola Marymount University and friends before working together on the show.
In the episode "Noshing and Moshing", a SOBE drink sign can be seen behind James Franco on the glass display case for beverages, when they are in the convenience store. SOBE didn't come out until 1996, which is sixteen years after the show is supposed to take place, in 1980.
"Smooching and Mooching" (episode 16, aired July 8, 2000): While at the Weirs' house during quiet hour, Nick breaks the silence by blasting "Tom Sawyer" from Lindsay's room, claiming listening to music is his homework. After a brief conversation about screwing around, Mr. Weir says "That drummer you're listening to, he's terrible...Neil Peart couldn't drum his way out of a paper bag...you wanna hear drumming? I'll play you drumming...I grew up with Gene Krupa and Buddy Rich". (Ironically, Harold Weir is played by Joe Flaherty, SCTV's "Count Floyd" who introduced the song "The Weapon" on the concert video screen during the Signals and Grace Under Pressure tours, as well as on the single release).
The trivia items below may give away important plot points.
In the beginning, Lindsay (Linda Cardellini) was meant to get together with Daniel (James Franco), not Nick (Jason Segel). However, this was changed, when it was decided that Daniel would be together with Kim (Busy Philipps).