Warning: This post contains spoilers from the Mare of Easttown finale.

After Mare of Easttown's fifth episode solved the mystery of who was kidnapping young girls, there was one unsolved mystery left: Who killed Erin McMenamin (Cailee Spaeny)? Although the finale gave us a couple of red herrings, the show eventually revealed the truth: Ryan Ross (Cameron Mann) had pulled the trigger, though it was an accident. After Ryan found out that his father, John (Joe Tippett), was sleeping with Erin — and was the real father of her child — he stole a neighbor's gun to scare her. But when she tried to wrestle it out of his hands, things went awry.

In the end, Mare (Kate Winslet) arrested Ryan, causing a massive rift with her best friend Lori (Julianne Nicholson), who is also Ryan's mother. (John had already taken the fall for the crime and Lori was hoping Mare would leave her son out of it.) EW spoke with Nicholson about the finale and Lori's involvement in the season's big twist.

ENTERTAINMENT WEEKLY: Did you know the ending going into this project?

JULIANNE NICHOLSON: I did know the ending going into it. I don't remember if it was before or after I accepted the job but it definitely was before we started filming. I did know what was coming.

But then Lori didn't know until John laid it all out for her there at the end. So is it easier to know something your character doesn't for most of the season or the other way around?

Interesting. I guess it depends. There's value in both. I'm a big proponent of, in life, when we have secrets, we try to play it cool. We don't want to give anything away. So as an actor, unless you're directed to, you don't have to act that truth or that secret because in life, all we want to do is cover. To me, it's an interesting idea to have the information but not reveal it yet. But I also love secrets and surprises.

Let's go back to the beginning: What initially made you want to do this show?

I've known Kate [Winslet] for many years and I've been a fan of hers for longer than I've known her, and so she called me and said, "I'm doing this show, you have to read it, you have to play Lori, Mare's best friend." I hadn't seen Kate in about 10 years or so. I read the first few episodes and I don't think I was allowed seven until after I'd accepted it but she did tell me most of the arc of the story. I don't think they gave away the details of who killed Erin until later but they talked around it and told me that in the last episode a lot happens for Lori and that the show ends with them and how important that friendship is to the whole piece. It's been so exciting to see how people have been responding to it. I don't know that I've ever been a part of something that there's been so much chatter about so that's been thrilling.

So what was your reaction when you found out the ending? Were you satisfied with it as the ending for this story?

Yeah, I was. There was a moment where I was like, "Did Lori do it?" It was interesting. I've had people say they hope she did do it and I was kind of like, "I hope she doesn't, I don't want to kill someone." [Laughs] I know it's acting but still. So it was satisfying for me to have so much to do around home and motherhood and friendship in that last episode in particular.

I thought Lori had done it for a bit, but I think that'd be too sad in terms of Mare's larger arc of healing.

That would be a lot. I think Mare would go off the deep end. Or maybe not, I'm sure she'd compartmentalize it.

In that final scene where Mare is comforting Lori, I found myself thinking about how Lori probably did this same thing for Mare when she lost her son. I love that it's ultimately a story about mothers and sons and friendship.

That's what they were aiming for or talking about from the start, that that's where they want this story to end and that felt really good to me too because I don't think you see that very often, especially in a whodunnit type show.

I was genuinely worried for a second that Lori wasn't going to forgive Mare.

Well, that's good! I was glad that they gave her a minute where she could be like, "F--- off! You've ruined everything and you didn't have to!" I still feel like she didn't have to. For as much as her moral compass is a little wonky, this is where you come down on the right side? I think I just felt protective of Lori and Ryan in particular. It's such a tragedy, so awful.

Did you actually get to shoot your final scene last?

No, we shot all seven episodes together. That scene of the two of us in the car was shot during the first couple of weeks of filming. Also, the scene where we go to visit Ryan in the end. It was all over the place, so part of the challenge of this show for everybody was figuring out where we were, what had happened, who knew what. Kate wouldn't film a scene without making sure everybody was clear with where we had been and what was still to come. It took a lot of people to keep that on track.

Was this role as emotionally draining for you as it looked?

Yes but it was also odd because the show was so big and there's so many people and different storylines that I did a lot of bouncing in and out. I found that more jarring than the work of it. But those are big stakes, so to try to play them as honestly as possible. It did feel exhausting.

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