Acting for Two: In Season 2, Elizabeth Henstridge plays both Simmons in her present characterisation and Fitz's hallucination of Simmons. The latter looks and acts more like her Season 1 self, but with occasional slips into Fitz's own personality.
Actor Allusion: Melinda May's mother states her agency hasn't fallen apart. She is portrayed by actress Tsai Chin, who played the undercover MI6 agent Ling in the James Bond movie You Only Live Twice. Also, they were mother and daughter back in the Joy Luck Club.
Ascended Fanon: One of the biggest cases ever. Coulson's death being undone in numerous fanfics and such spawned this entire TV show.
Nearly happened again by having Victoria Hand and Isabelle Hartley become a couple, which was a popular theory due to Hand's girlfriend in the comics being named Isabelle. That was until the writers realised the Unfortunate Implications attached to introducing two gay characters and then killing them both off almost immediately.
Ming-Na speaks fluent Cantonese, which she uses to show Melinda May's Bilingual Bonus. She grew up in British Hong Kong and Portuguese Macau before moving to America with her family.
Creator Couple: Jed Whedon (the younger brother of Joss Whedon) and his wife Maurissa Tancharoen are the showrunners of the series. note Joss himself has a significantly smaller role than most other Mutant Enemy projects due to being busy with The Avengers: Age of Ultron.
Dawson Casting: Inverted. The S.H.I.E.L.D. document about Skye being dropped off at her orphanage is dated 1989, which makes Skye a few years older than Chloe Bennet.
Enforced Method Acting: Brett Dalton noted on an interview with Marvel after "Turn, Turn, Turn" that he was informed that Ward would be revealed as a HYDRA agent near the ending of filming for "Yes Men", two episodes before the reveal. Assuming this was planned from the start, it means that he played Ward as an authentic good guy, making his deception more surprising when the time came.
Meanwhile, in an interview with HitFix before the season finale "Beginning of the End", Bill Paxton was informed he was Hydra right as he was hired, before anybody else on the set knew. So Paxton was allowed to play Garrett as a bad guy knowingly playing a good guy, and the rest of the cast were in the dark for quite a while... including Clark Gregg!
Fake Nationality: Englishman Ian Hart as Canadian Franklin Hall, Englishwoman Saffron Burrows as American Victoria Hand, and Irishwoman Ruth Negga as Americannote presumably Raina. Sunil Bakshi is likely supposed to be of South Asian-British descent based on his name, but he's played by Greek Cypriot-British actor Simon Kassianides.
Fandom Nod: In "The Only Light in the Darkness" it's revealed the orphanage Skye grew up at named her Mary SuePoots. Skye had previously been accused of being a Mary Sue by detractors of the character.
Fan Nickname: A few have taken to calling the series "Interns of S.H.I.E.L.D.", due to the mostly college age-looking cast.
The mysterious symbols that Garrett and later Coulson began doodling have been nicknamed "Kree math."
Getting Crap Past the Radar: When Coulson's team is reviewing Creel's abilities, they mention that he happens to feel some pleasure in changing his body to other materials. At that moment, the scene changes to showing him holding wood chips to turn himself into wood.
Col. Robert Hendry (Glenn Morshower) apparently survived getting blown up by Sebastian Shaw. He's also aged really well. Alternately, Special Agent Aaron Pierce went into the Marines and became a four-star general, just like in the Transformers films.
I Knew It: Half of the fandom knew Bobbi Morse was working for Coulson ever since they first mentioned she was 'Head of HYDRA Security'; the other half were hoping this would be the case and dreading the idea of it not being so. So, when it was leaked that this was indeed the case, people weren't surprised, though were very relieved.
People also guessed that Mockingbird would be Lance's often-mentioned ex-wife since about the second time he mentioned her; it helps that he mentioned she was friends with Hartley, who as a high-ranking member of SHIELD, would likely have known Bobbi, given Bobbi's apparent popularity within SHIELD.
Jossed: When fans saw that the pilot involved an African American man with superpowers, many assumed that it was Luke Cage. Whedon, however, assured that it wasn't him. For good reason, because he's actually the Villain of the Week who later becomes Deathlok.
Indirectly; fans had theories spring up about Isabelle Hartley being Victoria Hand's lover, due to Hand dating a woman named Isabelle in the comics. This is never confirmed in the show, but jossed by the creators as they realized what they did with both characters would lead to Unfortunate Implications.
Mean Character, Nice Actor: Agent Melinda May is possibly the most focused, ruthless, laconic, and stoic agent on Coulson's team who rarely cracks a smile, let alone laugh. Ming-Na Wen? A complete opposite as she is a very friendly, talkative, and relaxed lady who loves to interact with her fans. It's hilariously jarring watching behind the scenes footage or any of her candid videos for this reason.
Brett Dalton as Agent Grant Ward. While Agent Ward is a traitorous bastard, Dalton is the complete opposite as he is an extremely friendly, down-to-earth guy and a loving father to his daughter. Oh, and there's this too.
Name's the Same: "The Rising Tide", the hacktivist group Skye is part of, is also the name of a Real Life climate change activist group. The real life organisation Rising Tide is not pleased with the series using the name, since fans have confused them for some super hacker group (when in reality they're just a normal protest group).
"Melinda May" is also a song by Stephen Foster published in 1851.
Amy Acker, who also starred in Angel as well as several of Whedon'sotherprojects, plays Audrey the Cellist, the previously unseen love interest Coulson's been mentioning since The Avengers.
Reed Diamond, who played recurring character Laurence Dominic on Dollhouse as well as Don Pedro in Much Ado About Nothing, plays Daniel Whitehall, the seeming Big Bad for season 2.
Rewatch Bonus: Rewatching the first half of Season 1 with the knowledge that HYDRA is still active and Ward is a mole puts a lot of Ward's dialogue in a new light. Any time he talks about secrets, deception, or that time he offered a SHIELD academy scientist an opportunity with his secret unit, it feels like hints being dropped about the revelation in "Turn, Turn, Turn".
Rewatching the second season premier after learning that Simmons left between seasons and has been Fitz's hallucination makes his scenes even more heartbreaking.
Romance on the Set: Chloe Bennet began dating Austin Nichols after the latter guest-starred as Skye's no-good boyfriend Miles Lydon (hopefully that's not an omen).
Throw It In: In "Nothing Personal": Apparently, J. August Richards improvised patting Ward on the cheek after restarting his heart.
Melinda was originally going to be a white woman named Althea Rice. Ming-Na Wen gave such a strong audition that the character became Chinese and her name was changed to avoid the Unfortunate Implications.