David makes a comment to Snow about falling through a portal to Asgard. Josh Dallas was the original Fandral in Thor.
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.
The Danza: Young princess Eva is played by Eva Allan (also credited as Eva Bourne).
Dawson Casting: 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.
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.
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.
From LOST: Charles Widmore is Prince Charming's father; Claire Littleton is Belle; Hurley is Anton; Charlotte is the Wicked Witch of the West (Ben was right to try to shoot her...); and as of "White Out" Juliet looks to be the season 4 Big Bad.
Erica Sayers is Regina's controlling ambitious mother. So... Regina and Nina are siblings?
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.
Older Than They Think: 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 & 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.
In season 4, Merrin Dungey replaces Yvette Nicole Brown in the role of Ursula.
Playing Gertrude: 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.
Jennifer Morrison and Sebastian Stan, which is more creepy than the above example when their characters' interactions are considered. They broke up in 2013.
Shipper on Deck: Ginnifer Goodwin (Snow White), Emilie de Ravin (Belle), and Michael Raymond-James (Neal/ Baelfire all ship Swan Thief (Emma/Neal).
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.
Also, considering his overjoyed expression at their wedding, most fans have assumed that Archie/Jiminy ships Rumbelle...
Tinkerbell is a not so subtle shipper of Outlaw Queen (Regina/Robin)
Throw It In: During the Ruby/Granny fight in "Red Handed" August can be seen all, "These ladies crack me up." His actor was clearly trying not to laugh, 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.
Vindicated by Reruns: 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.
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.
Ariel and Aladdin were planned for the second season, but their appearances got "pushed back" for unknown reasons. (Ariel showed up in Season 3, but Aladdin has yet to appear).
Early drafts of the pilot episode had Charming killed off. This was changed after Josh Dallas auditioned and "blew them away" with his performance, prompting the creators to expand his role.
Early drafts also had Regina with three children instead of just Henry, but the identities of the other two are not yet known.
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.
Belle was originally intended for only one episode, but she was brought back by popular demand.
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.
Ruby had more plots planned but the ever-increasing character load in season 2 left them no way to feature her.