Speaking at PaleyFest, Ginnifer Goodwin explained that she knows how the spell - which traps fairytale characters in the real world town Storybrooke - can be broken.
"They explained to us what breaks the curse, which I'm so not going to tell anybody," she said, according to TV Guide.
"But once they told me, I kind of felt like a dumb-dumb because it makes perfect sense."
Meanwhile, Jennifer Morrison revealed that her character Emma will still refuse to believe that the curse exists, saying: "There hasn't been anything that can really legitimately prove to Emma that this insane idea could ever be possible. She's starting to realise there's something off and there's something wrong, but she's not sure what."
Once Upon A Time's executive producer Adam Horowitz added that fans will find out more about August (Eion Bailey), a mysterious character who made his debut in the show in January.
"Before we get to the finale, we'll be pretty clear on who he is, what he's doing, why he wants to do it and how he's going to do it," Horowitz teased.
Once Upon A Time airs on Sundays at 8/7c on ABC.
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