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.
Ming-Na Wen plays both May and Agent 33, who gets her May disguise fused to her face. Not only does she now play two roles, but one involves a quite complex makeup job for a while. And even when 33 gets the mask fixed, the voice remains broken so Wen continues to dub all her disguises.
Chloe Bennet also gets a brief turn at playing Agent 33, as she tries to seduce Ward using Skye's appearance.
And now Brett Dalton gets to play the trope in Season 3, as his character, Ward, is now being possessed by Hive.
In the Season 1 finale, Director Emeritus Nick Fury states that he plans on lying low for a while, but that he'll be "everywhere." Samuel L. Jackson may not be in MCU movies for a while, but he will be in other films.
A minor case; Fitz-Simmons both have the same birthdays and birth year as their actors, according to their ID badges and information given in the tie-in comics.note This slightly contradicts information given in the show, however; in a deleted scene from "Seeds", Simmons claims that Fitz is twenty-three days older than her, while Iain De Caestecker is actually three months younger than Elizabeth Henstridge. So, while Simmons definitely shares her actor's birthday (as can be seen on her ID card in the show), the same may or may not be the case with Fitz. However, this seems to have been the subject of a minor Retcon in later tie-in comics, which make a point of having Fitz share his actor's birthday for the sake of a running in-joke involving monkeys.
Brett Dalton and his character Grant Ward also share the same birthday.
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.
The familiar way Isabelle Hartley talks about "Vic" in a flashback hints this may have become unspoken canon.
Both Clark Gregg and Ming-Na Wen have martial arts training and use it in the show.
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.
In "The Dirty Half-Dozen", Chloe Bennet did all the fighting in The Oner without a stunt double.
Karate champion Mark Dacascos naturally gets to fight May.
Creator Couple: Jed Whedon (the younger brother of Joss Whedon) and his wife Maurissa Tancharoen are the showrunners of the series. note Joss himself had a significantly smaller role than most other Mutant Enemy projects due to being busy with Avengers: Age of Ultron, and eventually left Marvel after Age of Ultron was finished.
Creator Backlash: Joss Whedon thought that he could bring Coulson back from the dead for the series without cheapening what his death meant in The Avengers. Unfortunately, he found out that death in the cinematic universe isn't as cheap as it is in the comics and wanted to keep the movies and the show separate from each other, meaning Coulson can still be considered dead in the movies. Despite this, the show and the movies continue to influence each other, albeit tangentially through occasional references. Although it should be noted that Whedon did consider revealing that Coulson was alive to the Avengers in the film - he decided against it at least partially because it would take focus away from the main plot too much. Whedon also received some backlash from the other people working on the cinematic universe for creating a television show about S.H.I.E.L.D. right before they released the film where S.H.I.E.L.D. is destroyed.
Deleted Scene: The home release of Season 2 contains scene from the finale "S.O.S.", where a flashback to the day Daisy's birth was cut. Jiaying tells Cal to promise her that if anything happens to her, that he would protect Daisy. This is seemingly the promise that Cal mentions before killing Jiaying in the climax.
Development Gag: A slight one. The DVD Commentary for Iron Man 3 reveals there was supposed to be an Extremis explosion at Union Station. There is almost an Extremis explosion at Union Station in the pilot.
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!
Exiled from Continuity: According toWord of God, the show cannot use any X-Men, Fantastic Four, or Spider-Man characters since theirfilmrights are owned by Fox and Sony. They can't even use the word "Mutants", which is why similar terms like "Gifted" are used to describe characters like Scorch.note Consider for a moment that this all applies to a show that is made by Mutant Enemy Productions. After every episode is over, literally the first word on the screen is "mutant." Just enjoy that for a moment. It's reinforced every time someone insists psychic powers aren't real. In the X-Men films, it's clear everyone (at least in positions of authority) knows that they certainly are. The presence of superpowered individuals is likewise considered a new thing to the general public in the Marvel Cinematic Universe, whereas X-Men not only had very public displays of superpowers, there was a political campaign about registering people with them.
As of February 2015, Sony has struck a deal with Marvel to reboot Spider-Man and bring him into the MCU. Whether this deal extends to Spiderverse references in the TV shows is yet to be seen.
Englishwoman Saffron Burrows as American Victoria Hand.
Irish-Ethiopian Ruth Negga as Raina, who speaks with an American accent, and eventually reveals that she grew up in Thailand.
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.
Jiaying is implied to be Chinese, she's portrayed by Nepalese-Australian Dichen Lachman.
Gordon has a Brooklyn-sounding accent, but is played by British actor Jamie Harris.
Lincoln has an American accent, but he's played by Australian actor Luke Mitchell.
Eva Belyakov, a Russian, is portrayed by American actress, Winter Ave Zoli. Her daughter Katya is portrayed by Ava Acres, another American.
American actor Reed Diamond portrayed Werner Reinhardt/Daniel Whitehall, a German.
American actor Spencer Treat Clark plays German Werner von Strucker/Alexander Braun.
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.
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.
Also during Season 2, there were two theories being called out: firstly, that the mysterious aliens involved in the backstory were the Kree, thus setting up a plotline concerning the Inhumans; and secondly, related to that, that Skye was not a Canon Foreigner at all but someone from the comics proper, with the most prominent guess being Daisy Johnson/Quake, which by extension would make her Axe Crazy father Calvin "Cal" Zabo/Mister Hyde. By the mid-season finale, both these theories were revealed to be true.
After Winter Soldier, people went back over the MCU with a fine-tooth comb, looking for potential HYDRA infiltrators. "Among Us Hide..." explains a plot point in The Avengers. Remember that guy on the Council who Fury was talking too? He was HYDRA. He convinced them to launch the nuke, probably not to contain the threat, but to kill off all the Avengers before they could become a problem.
Quite a few fans correctly guessed that Andrew Garner was Lash.
When he first appeared, a few fans guessed that James was probably J.T. James, AKA Hellfire of the Secret Warriors. Then he gets his powers in his next appearance and this is confirmed.
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.
After directly revealing that Bobbi and Mack were indeed spying on the team for some reason, the very next episode took care to nip any theories of them being HYDRA in the bud, as one conversation goes a bit out of its way just so Bobbi can say "We're not HYDRA!"
Lying Creator: Early reports from the crew on Powers Boothe's character in the show, Gideon Malick, were that he would be a different character from the one he played in The Avengers. When he actually appeared in the show, it turns out they were one and the same.
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.
While Grant Ward is a traitorous bastard, Brett Dalton is the complete opposite as he's an extremely friendly, down-to-earth guy and a loving father to his daughter. Oh, and there's this too.
"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.
Brad Dourif was literally brought in just to sit in a chair for a few minutes, saying and doing absolutely nothing before getting killed, just so the audience would believe he was the Clairvoyant rather than Garrett's patsy.
Playing Gertrude: 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 (b. April 18, 1992). "Melinda" reveals she was born on July 2, 1988 - making Skye older than her portrayer by 3 years and over 9 months.
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.
Watching "Yes-Men" again after seeing "Melinda". In hindsight, this episode hints towards the trauma May faced from the Bahrain incident. A Welcome Wagon to investigate some person of interest (an Asgardian/an Index candidate), that ends up involving a person with mind control powers (Lorelai/Katya) that forces May to fight allies (Ward/"The Cavalry"). As another parallel to Bahrain, Lorelai has both powers of the Belyakovs (Eva's super-strength and Katya's mind control). May seems to realize this herself in the episode, as she bitterly notes during Fitz's presentation of the new I.C.E.Rs that it is difficult to distinguish friend and foes when mind control is involved (though the I.C.E.Rs being nonlethal make her willing to take them over a real gun).
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.
Trolling Creator: B.J. Britt being spotted on the set of Avengers: Age of Ultron seemingly spoiled the fact that his character, Triplett, would survive the events of Season Two, as it had already been revealed that some Agents of S.H.I.E.L.D. characters would appear in The Stinger. This being a Joss Whedon production, however, a few fans were banking on a double bluff, with the "accidental" sighting deliberately engineered by the writers. They were right: Trip gets Killed Off for Real in the mid-season finale, and in the end, none of the series' characters appear in Age of Ultron at all.
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.
Season 1 Episode 7, "The Hub" was written to exploit the real-life friendship of Brett Dalton and Iain De Caestecker - it is the first episode to air after the premier of Thor: The Dark World, and the following episode "The Well" covers the crossover elements. This along with the fact that Episode 10: "The Bridge" ends on a cliffhanger before the show returned the following year with "The Magical Place" strongly suggests that before "The Hub" was written, episodes 8-11 were meant to be aired as #7-10; ending the year 2013 with a two-part, mid-season finale.
It's been confirmed that Triplett was originally more of a Mauve Shirt - he was going to die in the Season 1 finale around the same time as Garrett did, but his popularity and likability with the production team (and fans) led to his getting a half-season reprieve before finally biting it. For that matter, Grant Ward is rumored to have been considered for death in the same season finale before they decided he had too much potential to be killed off that quickly.
A spinoff involving Lance Hunter and Mockingbird was planned for a 2016 release, but it was shelved in favor of giving Agent Carter another season. Nonetheless, there is a possibility that the spinoff could be revisited at a later time.
An extended scene on the Season 2 DVD with Bobbi and Agent Calderon implies that the Enhanced HYDRA asset that Weaver fought at the academy was about 8 ft. tall and had skin like steel. Presumably, it got cut because it was too close a reference to Colossus from X-Men.