'The Unforgivable' star and director reflect on the Sandra Bullock film's big twist and why it was 'really challenging' to adapt the story from the original miniseries

• Warning: Major spoilers ahead for "The Unforgivable."
• Aisling Franciosi said her favorite scene in "The Unforgivable" involves its big plot twist.

Warning: This post contains major spoilers for "The Unforgivable."

"The Unforgivable," a new film starring Sandra Bullock, hinges its emotional arc on a big plot twist — and Bullock's costar, Aisling Franciosi, told Insider that the scene revealing that twist was her favorite one in the movie, even though she wasn't in it.

The movie, currently streaming on Netflix, follows Ruth (Bullock), a woman trying to get back in touch with her younger sister, Katie (played by Franciosi as an adult and by Neli Kastrinos as a child) after spending 20 years in prison for killing the town's sheriff when he came over to the sisters' home, trying to evict them after their father's death, and Ruth shot him.

By the time Ruth leaves prison, Katie is in her mid-20s, having spent most of her life with her adoptive parents, who concealed Katie's past from her and allowed the young girl to repress her traumatic memories of the shooting.

As a result, Bullock and Franciosi only share one scene together in the movie, when Ruth and Katie finally meet once again as adults after Ruth's multiple attempts to contact her.

The reunion happens after flashbacks reveal the twist — that it was actually 5-year-old Katie who had accidentally shot the sheriff in a moment of distress, not Ruth, and that Ruth had taken the blame and gone to prison to protect her sister.

Franciosi told Insider she believes the bond between the sisters in the film is made especially "beautiful" by the fact that they don't actually meet until the very end — and that throughout the film Katie has been seeing Ruth in her dreams and feeling "strong emotions" about the mysterious woman who she doesn't even realize is her sister.

"That's part of what makes it really quite beautiful — that despite the fact that you don't see that much of them together, older Katie and Ruth, they are so connected," Franciosi said.

She added: "It just underscores the fact that when you have these bonds with people in your life who mean something to you, even with distance, even with time, they can still stay so strong."

Franciosi especially praised the performance of the actress playing the young version of Katie, Neli Kastrinos, in the flashback scenes, noting that while the truth of what happened with the sheriff "is a huge part of who Katie is," it didn't really affect her version of the character "in terms of plot," since she's not present when Ruth divulges the truth.

In a separate interview with Insider, director Nora Fingscheidt said it was "really challenging" both adapting the movie from the 2009 British miniseries "Unforgiven" as well as depicting Katie and Ruth's bond on-screen with such little shared screentime between the two as adults.

"In order to still keep the connection between Ruth and Katie, we created that memory layer, because Ruth is a character who doesn't speak a lot," Fingscheidt explained.

She added: "We needed to find visual choices that Ruth and Katie share even if it's buried under Ruth's silence and Katie's amnesia. There is still a past life that connects them."

As for Franciosi, she said she's still wondering about the somewhat ambiguous happy ending and what happens after Ruth and Katie finally reunite.

"I do sometimes wonder what happens after the movie ends. Does anyone tell Katie the truth or not?" Franciosi told Insider. "But in terms of how I played that moment, it was actually fine, because Katie doesn't know that's what happened, and she doesn't find out over the course of the movie either."

"The Unforgivable" is currently streaming on Netflix.