Creators said as much about Merrin Dungey's casting as Ursula from The Little Mermaid, as she does not resemble the character much. But they were inspired to cast her because of her performance in Alias.
Likewise Joanna Garcia is a full eighteen years older than Disney's Ariel, being 33 when she first played her. But it's hard to imagine an actress who can capture Ariel's optimism, clumsiness and quirky attitude better than she can.
To a lesser extent, Sarah Bolger looks nothing like Disney's Princess Aurora but her performance was very well received.
Caroline Morahan is far too young and pretty to be Brave's Queen Elinor but turns in a spirited and heartwarming performance nonetheless.
According to Rebecca Mader, she expected to do an American accent to play Zelena - since the Wicked Witch comes from an American fairy tale. But the creators insisted on using her natural accent, and she was the first choice for the role.
Christie Laing, who plays Marian, didn't know that Marian was actually Zelena in disguise in season 4 until "Heart of Gold".
Eion Bailey auditioned for August pretty much knowing only what was in the script for his first appearance. He joked that the creators went to great lengths to hide the twist that he was really Pinocchio for as long as they could from him.
Otherwise averted with David Anders, who only signed on when he was told his character was really Dr Frankenstein. Likewise Meghan Ory was told that Red would turn out to be the wolf a few episodes before it actually happened.
Colin O'Donoghue had no idea Hook was a second Dark One for the first half of Season 5. Quite justified in this case as Hook didn't know either.
David makes a comment to Snow about falling through a portal to Asgard. Josh Dallas was the original Fandral in Thor.
Eion Bailey previously played a writer in Band of Brothers. His character in that went on to write a book about sharks in real life and ended up lost at sea - which is interesting, given that Pinocchio was nearly eaten by a whale.
Actor-Inspired Element: Elizabeth Mitchell suggested that Ingrid be barefoot, as she found it hard to walk in the woods while wearing high heels.
Ascended Fanon: Due to how little the final film bore to the source material, many fans of Frozen had theorized that Andersen's Snow Queen existed in the Frozen-verse as a separate entity than Elsa, another fan theory being that Elsa's powers were tied back to the former. Once appears to be going with an idea similar to this, with Elizabeth Mitchell portraying the Snow Queen as a separate character from Georgina Haig's Elsa. Both theories were ultimately used, with the Snow Queen turning out to be Elsa's aunt.
As noted above, David Anders signed on when he was told he was actually playing Dr Frankenstein.
Rebecca Mader was a huge fan of the show and claims she screamed in public when she was offered a role. She agreed before she even knew which role it was.
Merrin Dungey likewise immediately agreed to move to Vancouver to film and dye her hair blonde, as she had been a big fan of The Little Mermaid - and jumped at the chance to play Ursula.
Georgina Haig had a reaction like this to being cast as Elsa. She describes walking around in the costume as "like being Santa Claus".
The Danza: Young princess Eva is played by Eva Allan (also credited as Eva Bourne).
Billing Displacement: Raphael Sbarge (Archie) during season 1 and Michael Socha (Will Scarlet) during season 4 are in the credit opening as main characters despite being actually secondary. Eoin Bailey also qualifies as although his role in season 1 did become crucial in later episodes, he was only in half of them and ultimately functions more as a supporting character; like Sbarge, he was billed as recurring/guest thereafter.
Robert Carlyle hated that Neal was killed off, feeling Rumple's character lost his main motivations afterwards.
Amy Manson not-so-subtly retweeted a couple of fan tweets complaining about the Season 5 winter finale.
Ginnifer Goodwin has also subtly hinted that she's not happy with the direction Snow's character went.
Jennifer Morrison has publicly disagreed with the producers' character interpretations on several occasions, most notably saying that Emma taking the darkness into herself was about saving the whole town, rather than just Regina.
Michael Socha has been quite vocal that he didn't enjoy his time on the show because even though he was contracted for a full season he really didn't do anything while he was on set.
Creator's Favorite: Edward Kitsis and Adam Horowitz reportedly said that Regina was their favorite character.
Elsa is 23 years old and is portrayed by 29 year old Georgina Haig. This is just slightly less egregious than Frozen, where a 21 year old Elsa is voiced by 42 year old Idina Menzel.
Whilst played straight in Frozen (where a then-18 years old Anna is voiced by a 33 year old Kristen Bell), OUAT makes it a minor trope, since Anna is 20 years old and is played by a 22 year old Elizabeth Lail.
Kristoff is 23, and portrayed by a 29 year old Scott Michael Foster.
Nicole Munoz was twenty when she played Lily at 16. And since the show is a little vague on how much time has passed since the first episode, the adult Lily is this too. There's a seven year age gap between Agnes Bruckner and Jennifer Morrison, despite their characters supposedly being the same age.
Ruby is implied to be late teens or early twenties. Meghan Ory was nearly thirty when she first played her.
Peter Pan's age is vague, but he's meant to be an eternal little boy. Robbie Kaye was eighteen.
Defictionalization: The exterior shots of Storybrooke's main street are filmed on a real road in British Columbia. The signs on certain shops are covered with a Storybrooke equivalent during filming and removed afterwards. However, some shop owners let the production company keep the signs up permanently partially due to this trope and partially to make less work for the film crew (to erect and tear down the signs every week). The producers also leased two units just for the storefronts. These are kept the same year round, since they were never real shops in the first place.
Deleted Scene: The Season 5 winter finale featured a scene wrapping up the loose ends of the Camelot storyline, but it was ultimately cut from the episode.
Dueling Shows: Was initially this with Grimm, an NBC show that debuted around the same time that also focuses on fairytale characters existing in the real world. It soon became clear both shows were going in different directions. Grimm focuses on more obscure characters and creatures from the many, many lesser known Grimm fairytales, as opposed to the more famous and iconic ones in Once.
The Queen's fans are called Evil Regals, Snow White's are the Fairest, the Seven Dwarves' are Team 7, Prince Charming's are Charmers, Ruby's are Little Reds, and Belle's are Beauties. Emma's fans were originally called "Team Emma" though "Ugly Ducklings" is becoming more popular.
Mr. Gold perfectly remembers being Rumplestiltskin. Most fans probably guessed it from a mile away, though The Reveal was no less satisfying.
Surprise (or not)! Cora is the Queen of Hearts after all.
Several people guessed as early as the first season that Henry's father (Neal) is Baelfire.
Fan boards had joked since Frozen hit theatres about how it would translate to that universe. Guess who just showed up...
Many had theorized that Elsa wasn't the actual Big Bad of Season 4 and that the Snow Queen was the real villain. They were right.
A smaller case. When Merrin Dungey's Ursula appeared in Season 4 (and was referred to as "the sea witch"), a few assumed that she wasn't the sea goddess that Eric's kingdom worshipped. "Poor Unfortunate Soul" confirmed that she was just named after the goddess - though it's not confirmed whether that was her at the end of "Ariel" or the true goddess.
Lily being the daughter of Maleficent was so obviously guessed by everyone it would have been a bigger twist if it hadn't turned out that way.
Henry's middle name being Daniel was a popular theory.
A smaller one brought on by The Reveal that Zelena was really Marian in disguise. Two episodes later revealed she was pregnant with Robin's baby, which had been tossed around by fans.
The Evil Queen is (or was) — well, evil. Lana Parrilla, on the other hand, is an incredibly nice, warm, lovely human being who adores her fans and fellow cast members, is a second mom to Jared Gilmore (Henry), and has embraced her "Evil Regals" with open arms.
Rumplestiltskin is... not nice. Robert Carlyle, on the other hand, is by universal acclaim the nicest person on the set. Given the temperaments of most of the rest of the cast, that's saying something.
Hook was not a very nice person initially. Colin O'Donoghue, on the other hand, is incredibly polite, kind, and is completely Adorkable.
Zelena is, of course, wicked. Rebecca Mader is a kind, geeky, Adorkable woman who is always ready to answer her fans' questions on her Twitter, and was a big fan of the show before even being in it.
Peter Pan is one of the most horrible characters on the show. Robbie Kay is very friendly and laid back.
Cora is the reason Regina is evil and makes her look nice at times. When Barbara Hershey had a scene in "Sisters" that required her to shout at the actress playing Young Regina she hugged the girl afterwards.
Little Red Riding Hood hunting a werewolf that terrorises her village every month at the full moon? That's the plot of a film called Red Riding Hood with Amanda Seyfried as the heroine. In both movies the love interest is a suspect and both movies call him Peter to boot (possibly as a Shout-Out to Peter and the Wolf). In both stories Red's father was a wolf. Except in the film he's the main antagonist while in the show he's passed it on to Red herself.
Ditto for True Love's Kiss working as a symbol of family love. It was originally scripted for Snow White and the Huntsman - where the Huntsman was conceived as a much older man. The kiss that wakes Snow White up was meant to symbolise a father's kiss. Ultimately things were changed, Chris Hemsworth was cast and the Huntsman became a love interest instead.
It seems to be intentional. In Season 1, the Queen of Hearts is played by Jennifer Koenig. In Season 2, she's played by Barbara Hershey when she's revealed to be Cora. It should be noted that even when she was played by Jennifer Koenig, Barbara Hershey supplied the voice, which was hard to make out due to her speaking only in whispers.
In season 3, Sean Maguire replaces Tom Ellis in the role of Robin Hood. It's later justified when a flashback taking place before his first appearance shows Robin stealing a glamour charm from Zelena and using it to hide from Rumple, whom he double-crossed. He says that the charm will make him appear to be someone else in the eyes of his enemies, so Rumple doesn't recognize him when they meet next.
In season 4, Merrin Dungey replaces Yvette Nicole Brown in the role of Ursula. Subverted when the episode Poor Unfortunate Soul reveals that they're separate characters: Dungey's character was named for Brown's.
In season 5, Wes Brown replaces Sage Brocklebank as Gaston.
The Other Marty: Paula Marshall was first cast as Maleficent, but was later replaced by Kristen Bauer von Straten.
Rose McIver usually plays women with tragic backstories, or who have gone through Break the Cutie in some way. Tinkerbell is similar but recovers from her damage and reverts to The Cutie pretty quickly.
Annabeth Gish, who plays Red's mother is only eleven years older than Meghan Ory.
Sally Pressman who plays Helga is two years younger than Pascale Hutton - her younger sister Gerda.
Frances O'Connor, who plays Belle's mother, is only fifteen years older than Emilie de Ravin.
And last, but far from least, Lana Parrilla is only one year older than Ginnifer Goodwin. This is fine in the first few episodes, since Parrilla is only known as the stepmother. But Regina's backstory episode has Parrilla play the young Regina, with a child actress playing Snow at eleven. This means there must be at least an eight year difference between the two characters.
Brave's Queen Elinor is played by an actress that looks a bit young. Granted she would have been fine for the events of the film. But here Merida and her brothers are at least ten years older.
Jennifer Morrison (Emma) and Colin O'Donoghue (Hook) both ship Captain Swan (Emma/Hook). Josh Dallas (Prince Charming) seems to be on board as well.
Throw It In: During the Ruby/Granny fight in "Red Handed" August can be seen barely containing himself from laughing. Eion Bailey really was trying not to crack, but it was so funny the scene was added.
Unreliable Expositor: Emma claims to have been 18 when she had Henry, but he was already 10 when she turned 28, meaning she had to have been 17 when she gave birth to him.
In Manhattan (which at the most is 6 months after the pilot) Henry is 11, making Emma 16 when she was pregnant.
A common occurrence through DVR ratings. Example: the first half of Season 3's Neverland arc (episodes 1-6) was a ratings success on all fronts, but the second half not so much. A rerun of its final episode, however, brought its ratings score up to standard.
Similarly, even though the live ratings have plummeted recently, it has become one of the top shows that people rent and binge-watch on Netflix.
Apparently they wanted Lady Gaga as the Blue Fairy.
The Sheriff was meant to be Sherlock Holmes, and his curse in Storybrooke was that there was no mystery to solve. The plan fell through due to an issue with acquiring the rights. Not to mention the fact that there are already twoSherlocks on TV right now.
Hook was supposed to have a larger role in the second half of season two, but it had to undergo a major rewrite due to Colin O'Donoghue breaking his leg. The writers had to cut almost all of Hook's scenes.
Regina and Henry's relationship from season 2 onward is only a result of Lana Parrilla (Regina) pushing the writers for it.
The same is true for Regina and Robin's relationship, somewhat...she only pushed for a romance for Regina, the choice of Robin Hood for the love interest is the writers' doing.
Belle was originally intended for only one episode, but she was brought back by popular demand. Similarly, Anton the Giant was originally going to only be in his nameless debut, "Tallahassee", but the staff liked him enough to bring him back and expand upon him in "Tiny" and "Lacey". Another similar case is Blackbeard, whose death in his debut, "The Jolly Roger", was later retconned because the writers really enjoyed what Charles Mesure brought to the role and thus wanted to use the character in more stories.
Captain Hook was originally supposed to appear in season 1, but the writers were unable to acquire the rights to Neverland in time.
Dracula was considered as a character for season 1, but they went with Dr. Frankenstein instead.
Ruby had more plots planned but the ever-increasing character load in season 2 left them no way to feature her. Season 5 re-introduces her.
Freeze-Frame Bonus: In Season 4, a quick glance around Rumplestiltskin's castle shows that he hold Mjolnir, aka Thor's hammer. As noted above, David references Asgard in another episode.