The Oscar nominee, 40, revealed during an appearance on BBC Radio 2 that she wore a "prosthetic butthole" while shooting a nude scene for the historical drama.
"This movie, it needed to be graphic," Seyfried explained, noting the intensity required to portray religious leader Ann Lee, founder of the Shakers.
Although the scene called for full vulnerability, Seyfried clarified that she was never completely exposed on set.
"I was pregnant and naked, but I wasn't naked at all," she shared, adding that she also wore a merkin during the film's fiery final sequence. "At the end of the movie, I'm standing in front of a burning building with just a merkin. I felt so free."
Seyfried emphasized that audiences won't actually see the prosthetic in the finished cut. The addition, she said with a laugh, was simply a precaution, a "just in case" measure to ensure realism if needed.
The lighthearted reveal quickly sparked buzz online, with fans praising the actress for her unfiltered honesty and offbeat sense of humor during the film's press tour.
Still, the role itself was no joke.
Seyfried previously described the project as "daunting," explaining that she intentionally seeks out work that challenges her.
"I feel like I have to keep choosing things that terrify me, within reason," she said in a past interview. "I need to sink my teeth into something that is really scary because I know I can do it, and I know I'll be better for it."
Her bold performance paid off. The film earned a 15-minute standing ovation at its Venice premiere and secured Seyfried nominations at both the Oscars and Golden Globes.
While she lost the Golden Globe to Rose Byrne, Seyfried has maintained that winning isn't everything.
"Would it be great? Of course," she said earlier this year. "But it isn't necessary."
