Once Upon a Time star Jennifer Morrison recently revealed why season 5 has been quite "exhausting" for her to film so far.

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Morrison, who plays Emma Swan on the ABC fantasy series, spoke with PopSugar about portraying her character with a darker twist now. Emma sacrificed herself to the darkness in the season 4 finale, meaning she will turn into the Dark One. Now that Emma is free of responsibility to do the right thing, Morrison has to play her quite differently.

She explained that it is actually draining to be in an extended mode of anger throughout the day.

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"The first couple of days of doing it, I was so exhausted!" she admitted. "When you get angry or angsty in real life, it happens in a second. It comes out, and then it's over. But when you're angry as a character, for 16 hours a day, you have to sustain that level of anger and angst for 16 hours, so you wake up the next morning and go, 'God, I didn't know I had those muscles! I didn't know I was carrying all this tension for so many hours!' But it's the most rewarding exhaustion. There's nothing more fun as an actor than to feel like you're exhausted for all the right reasons. I'm really enjoying it."

Morrison noted that everything has had to be changed as Emma takes on a new darker role, with her clothes changing, her attitude changing and more.

Morrison also spoke to the overall theme of the season, which is that evil is not born, it is made. She said, "It's so easy to justify something when you don't know the domino effect of how far that can take you in life. In that perspective, I do think that darkness or evil or bad decisions are coming from something that's circumstantial or made, not something that is born."

Once Upon a Time season 5 premieres Sunday, Sept. 27 at 8 p.m. ET/PT on ABC.

Tags: Once Upon A Time television ABC