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 canceled, 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 canceled. 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 canceled, and a season finale if the show would've gotten picked up for a second season). Several episodes later, it was canceled.
Nearly everything that happens to the characters happened to 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.
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.
Where are they now? Linda Cardellini recently starred in Avengers: Age of Ultron. James Franco went on to become a huge movie star, even receiving an Academy award nomination for his work in 127 Hours. Seth Rogen is now a big comedy man and recently starred in Neighbors and Neighbors 2: Sorority Uprising. Jason Segel went on to star in the smash hit How I Met Your Mother as well as comedy movies such as I Love You Man. Martin Starr is now probably mostly known for his role as Gilfoyle in the HBO show Silicon Valley. John Francis Daley now stars in Bones and co-wrote the movie Horrible Bosses. Samm Levine mostly has guest appearances in TV shows but also starred in Inglorious Basterds.
During the opening credits, you can see James Franco and Jason Segal vigorously rubbing their eyes to give themselves that "burn-out" 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.
"Smooching and Mooching" (#113, 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).
James Franco, John Francis Daley, and Martin Starrr are all linked to the Spider-man franchise. Franco plays Harry Osborn in the Raimi trilogy. Daley wrote the screenplay for the upcoming Spider-Man: Homecoming, Starr plays a role in that same film.
When Daniel takes Ken and Nick to the punk club, Ken and Nick make fun of Daniel's new punk hairdo. Jason Segel who plays Nick, has a role in SLC Punk! (1998) in which he is friends with two guys who also have punk hairdos.
The trivia item below may give away important plot points.
In the beginning Lindsay 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).