We've been promised a dark "Sherlock" Season 4, and that should come as no surprise after the events of Season 3 -- especially the unveiling of Mary Watson's huge secret in Episode 3: "His Last Vow."

In a new featurette released Wednesday by PBS, Amanda Abbington opens up about her character's big twist and what made it so interesting.

*If you haven't seen the episode, there are spoilers ahead.*

"Sherlock" fans weren't the only ones shocked to find out that Mary was actually an assassin -- and one capable of shooting the titular detective (Benedict Cumberbatch) at that. The actress herself only discovered the secret while reading through Episode 3 with her co-star Martin Freeman. The twist "pulled the rug out" from underneath her, she says, but she ultimately liked how it worked out.

"I think the payoff in Episode 3 is bigger and more powerful because you didn't know that that's what she was going to be," she says in the video.

Unfortunately, she didn't reveal any of the surprises ahead in "Sherlock" Season 4, but we'll see them play out when the first episode premieres on Jan. 1 at 9 p.m. ET/PT on Masterpiece on PBS.

[via: Masterpiece PBS/YouTube]