The producers had a choice between keeping Slade Wilson as an American, like in the comics, and casting Maori actor Manu Bennett in the role. They took the second option and turned Slade into an Australian spy.
Stephen Amell is also a case of this, if only because of his hair colour - in the present day he looks mid-to-dark brunet as Oliver (like Stephen is in real life), but in flashbacks where he has longer hair and a beard both are dyed a dirty-blond (making him resemble the comic's Oliver Queen a lot more). Because it would be hard to die his short-cropped hair (let alone his stubble), and because Amell portrays the character so convincingly, most people treat this as Acceptable Breaks from Reality and are generally not bothered by it... Worth noting is that Oliver's probably supposed to have a shade of blond in the show as well (that simply looks darker in the present thanks to lighting and shortness), because both his parents are blond and Thea is really Malcolm Merlyn's biological daughter, thus explaining her own brown hair.
Willa Holland (Thea) is pretty much playing her character from Legion - a rebellious teenage daughter to an upper-class family.
Thea describes Roy Harper as 'Abercrombie-looking'. Guess which company his actor modeled for? Revisited in "Broken Dolls", where Sin also keeps calling him Abercrombie. It becomes her Affectionate Nickname for him.
A flashback in "Heir to the Demon" reveals that Mrs Lance was a Professor. Alex Kingston has played a Professor before.
Tatsu is handy with katanas, just like Yukio, the character Rila Fukishima played in The Wolverine.
The Spartacus alumnae also get this. Katrina Law's character is still an Archer ArchetypeAction Girl, while Manu Bennett and Nick E. Tarbay's characters are (as of Season 3 Episode 8) now cellmates (in Spartacus, the gladiator quarters is very much like an "open prison").
Approval of God: Stephen Amell has "Meme Monday" on his Facebook page, where he reposts memes of the show that he finds funny. He's also done some bizarre one-offs, such as "Lightsaber Saturday" where he does the same sort of thing only with a specific theme.
A mild example, compared to others — Stephen Amell has quite happily gone on record with his displeasure over the marketing for Season Two...namely that the posters highlight the ensemble cast, but leave out Paul Blackthorne and Susanna Thompson. He similarly criticized WB for announcing plans for a Flash film and a new actor the day after The Flash posted strong second-week ratings, showing true staying power, feeling it undercut both the show and Grant Gustin.
Caity Lotz was understandably not pleased with her character's treatment in Season 3, being killed off in the premiere and then strung along for more appearances anyway. She did two (as a fake voice, and a hallucination), and then refused to do any more.
Dawson Casting: Averted for the most part, as most of the young adult characters are portrayed by actors 2-4 years older and so convincingly pass themselves as those ages; Katie Cassidy (Laurel Lance) and Emily Bett Rickards (Felicity Smoak) are in fact younger than their respective characters. The flashbacks, on the other hand, cannot avoid this pitfall because they also function in chronological order but are always 5 years behind the present-day scenes: 23-year-old Willa Holland stands out when portraying a 15-year-old Thea Queen in Season 3's "The Return".
Dyeing for Your Art: Katie Cassidy lost a significant amount of weight during the production of season two, to better convey Laurel's addictions to painkillers & alcohol. The following season, Katie bulked up in the gym as Laurel suited up as Black Canary.
Diggle's unborn child has already been named Diglett.
Sally the Salmon Ladder.
Deathstroke and Brother Blood's Mirikuru-empowered minions are referred to as the Deathstrokes, or some variant of, by many fans, though aren't actually named in the show itself. One of the alternative names is 'The Blood Cult'.
Laurel has been given nicknames such as Blackmail Canary (due to her habit to use blackmail on Kate Spencer during the latter half of Season 2) or Fakanary (thanks to some people considering that Sara is the show's true Black Canary)
Stephen Amell himself has been nicknamed 'Captain Amell' or 'The Captain' by Olicity supporters, thanks to his enthusiasm for Oliver and Felicity's relationship. Other parts of the fandom are now picking it up out of pure affection.
Follow the Leader: The success of Arrow not only spawned a spin-off, but prompted Fox & NBC to launch their own shows based on DC Comics characters, and Netflix to partner with Marvel on a series of shows about street level heroes in the Marvel Cinematic Universe. It can even be argued that Arrow's success is what led to NBC finally going ahead with the revival of Heroes. In fact, if the deal CBS made for a Supergirl series goes through, we'll have gone from no comic-book shows to one on every single broadcast network.
Hey, It's That Guy!: Let's start off with saying that somebody on Arrow must really have liked Spartacus, no less than three actors from that show have shown up. Manu Bennett, Cynthia Addai-Robinson, Katrina Law, and now Nick Tarabay all appeared on Spartacus before moving here (Liam McIntyre will also be appearing on The Flash (2014)). Especially interesting is the fact that, unlike other examples of actor-overlap,Spartacus was filmed half a world away in New Zealand.
Poor Kaitlin's family members have a nasty habit of dying.
It's worth noting that Paul Blackthorne's best-known role is magician Harry Dresden, and he plays a cop. John Barrowman's most famous role is him playing a sort of secret agent-slash-cop, and he plays a man whose codename is "Magician".
A number of people suggested that due to her brown hair not being a trait inherited from two blonde parents, Thea wasn't Robert and Moira Queen's daughter. It was revealed in "State v. Queen" that Thea was conceived after an affair between Moira and Malcolm Merlyn.
Malcolm is Back from the Dead. Okay, technically he never died in the first place, but the point stands.
As soon as Sara asked Oliver "What happened to Slade?" without mentioning Shado, people correctly assumed she was killed.
Slade's return. Given the character's popularity, most fans knew he wouldn't be killed like he was. Also, since the character is known for being a villain, as soon as they mentioned what Ivo was looking for and what Brother Blood was doing, many worked out he would be used on Slade and that he was the Big Bad well ahead of the reveal.
Malcolm Merlyn being involved in Sara's death in Season 3; not as obvious and many had their doubts, but some still expected The Reveal just the same.
John Barrowman as Malcolm Merlyn. Hold most of the mean, though. He has said it's fun to play a bad guy for a change.
Oliver Queen, at least early on, was cold, distant, and a bit of a jerk. Stephen Amell, on the other hand, is incredibly friendly and interactive with fans.
No Stunt Double: Zig-Zagged - Stephen Amell does as much of his own stunts as he's allowed, including the infamous salmon ladder which Amell can pull off in real life, with some of the more complicated/dangerous stuff being done by a stunt double.
One of Us: Head writers Guggenheim and Kreisberg are big comic fans and love continuity gags.
Real Life Writes the Plot / The Cast Showoff: Summer Glau's character, Isabel Rochev, was not initially going to be a villain, rather a red herring. However, Summer was dissatisfied about being in a superhero series without getting in on the action, so Isabel Rochev became the Ravager.
Mr. Blank was originally supposed to be the DC Comics villain Onomatopoeia. However, as Onomatopoeia only speaks in, well, onomatopoeia, they couldn't make it work without him coming across as mentally disabled (since sound effects sound far different in real life compared to comic books).
The twentieth episode of Season Two was originally going to be a Back Door Pilot for a potential spin-off focusing on The Flash. Based on both the strength of Grant Gustin's portrayal of Barry Allen in "The Scientist" and "Three Ghosts" and the fact that using the twentieth episode as a Back Door Pilot would interrupt the show's momentum as it built up to the season finale, the decision was made to make it a standalone pilot.
In early drafts of Season Two, Sara was originally going to take the role of Ravager, but eventually ended up as the Canary. Instead, the role of Ravager was given to Isabel Rochev.
The character who was going to buy out Queen Consolidated was originally going to be Ted Kord, a character whose existence was already implied through the appearance of Kord Industries. However, DC enacted an embargo on him due to "other plans", so the creators instead went with Ray Palmer. This means that the A.T.O.M. suit functions more like the Blue Beetle or Iron Man suits than it does its comic counterpart.
Before Matt Nable was cast, Liam Neeson said he'd play the show's version of Ra's Al-Ghul "in a heartbeat". Word of God states that after learning this, the network approached Neeson on the off chance he had the time, but they were unable to reconcile their schedules.