Original show: None of the characters appear in every episode, though the iconic eight characters appeared in every episode in season 3 (the only season they actually did). Kelly, Steve, Donna, and David were the only ones of the original 11 main characters to appear in every season as a regular, with Donna (unsurprisingly) appearing the most, missing just two episodes in the first season.
Reboot show: Naomi, Annie, and Silver appeared in every episode, Adrianna missed one, and Dixon didn't show up in three. Liam appears in every episode from his debut towards the end of season 1; Navid misses one to five episodes per season; Ethan appeared in every episode in the first season, before moving to Montana; the rest of the cast missed numerous episodes before being placed on buses in droves.
The Danza: Ryan Matthews is played by Ryan Eggold.
Riley is played by Riley Smith.
Dawson Casting: In the extreme, to the point where this trope is often called "90210 Syndrome" outside of this wiki, since the extremeness makes Dawson's Creek look like they cast age-appropriately.
The original cast members were Jason Priestley (21), Shannen Doherty (19), Jennie Garth (18), Ian Ziering (26), Gabrielle Carteris (29), Luke Perry (23), Brian Austin Green (17), Doug Emerson (16), and Tori Spelling (17). Plus, James Eckhouse (35) and Carol Potter (42), who definitely didn't play teenagers.
The reboot's original teen cast consisted of Shenae Grimes (18), Tristan Wilds (19), AnnaLynne McCord (21), Dustin Milligan (23), Jessica Stroup (21), Michael Steger (28), and Jessica Lowndes (19). Steger was older than Ryan Eggold (24), who portrayed the kids' teacher. Plus adding from season 2, Trevor Donovan (30) portraying a 16-year-old.
Directed by Cast Member: Jason Priestley directed 15 episodes, James Eckhouse directed three episodes, Jennie Garth directed two episodes, Luke Perry also directed two episodes, Brian Austin Green directed an episode, Tori Spelling also directed an episode, and Ian Ziering also directed an episode.
Dye Hard: Silver is a natural blonde as shown in the parent shown, but is a brunette for the entirety of the series save for when she briefly dyed it red.
The original show: In an interview given shortly after her Best Actress Oscar win for Boys Don't Cry, Hilary Swank (Carly, Steve's season 8 girlfriend) revealed that she had been fired from the show because the producers didn't think that Carly was connecting with the show's audience.
The reboot show: Season two drops Ethan and focuses a lot more on the new characters rather than the old cast. That's cleverly done for Kelly's mother's funeral as it's not shown and Silver comments that a lot of people that knew her but she didn't know attended.
Fake Brit: Blair Redford who plays Oscar is not British.
Fake Nationality: Michael Steger is a mix of several nationalities but Iranian(or anything Arabic) is not one of them.
I Am Not Spock: Jamie Walters actually hated his role as Ray Pruit, Donna's abusive boyfriend, claiming that the character was impeding his potential music career by making people refuse to buy any of his albums or concert tickets on the grounds that they actually thought he was like Ray in real life.
The Other Darrin: Rather than bringing back Mercedes Castner to play Silver, she was played by Jessica Stroup.
Playing Gertrude: James Eckhouse (Jim Walsh) was only 14 and 16 years older than his onscreen children Jason Priestley (Brandon) and Shannen Doherty (Brenda), respectively. He was only six years older than Gabrielle Carteris, who portrayed Andrea, his (onscreen) children's high-school-aged friend.
Reality Subtext: The Sequel Series had a particularly harrowing example for Anna-Lynne McCord. During filming of the scene where Naomi is raped, McCord's castmates simply thought that she was a really good actress, not knowing that she had actually been sexually assaulted herself when she was eighteen. Her breakdown and tears during that scene? She wasn't acting — she was having legitimate flashbacks to her rape.
Matthew Perry appeared as a one-shot character in the episode "April Is The Cruelest Month". He would go on to play his famous role as Chandler Bing three years later.
Vivica A. Fox, before her breakthrough role in Independence Day appeared in "Ashes to Ashes". Also Eugene Byrd, who portrays squintern Clark Edison on Bones, appeared as Fox's on-screen brother in the same episode.
Jessica Alba, pre-Dark Angel, in the episodes "Making Amends" and "Nature and Nurture" (both in season 8).
Paul Johannson, before One Tree Hill, itself a show about teens who looked in their 20's and...had problems.
In the new series: Stephen Amell played a presumed dead guy who returns after being lost at sea. One year later, he plays the same type of character as his breakthrough role, Oliver Queen on Arrow.
Romance on the Set: Brian Austin Green (David) had a relationship with Tiffani-Amber Thiessen (Valerie) for a year, even moving in together before breaking up. Green later had a son with Vanessa Marcil (Gina), though they too separated.
Unintentional Period Piece: This was the first of the many, many iconic teen dramas of The '90s, and the first of many to fall into this trope. It also does a great job at keeping up with the times. Season one looks straight out of the 80s while season five and later look straight up 90s.