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Sherlock Showrunner Steven Moffat Explains That Big Finale

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Spoilers ahead for the season finale of Sherlock.

The season-four finale of Sherlock felt like something right out of the Saw playbook, with Sherlock, John, and Mycroft teaming up to figure out how the estranged Holmes sister, Eurus, managed to escape her Alcatraz-esque asylum for “uncontainables” and wreak havoc on their lives. She most certainly did escape, and after programming every prison guard to work for her, Eurus inflicts some major psychological torture on the trio, revealing herself to rightfully be Sherlock’s most frightening adversary ever. (Not to mention, Moriarty valiantly returns in a flashback sequence and helps Eurus from beyond the grave. Welcome back, baby!) Eager to learn more about the action-packed finale, which will screen in select American movie theaters on January 18, Vulture spoke with Sherlock executive producer Steven Moffat to discuss the creation of Eurus Holmes, Sherlock’s humanity, and why “The Final Problem” would be a fitting series finale.

As someone who has a moderate fear of flying and is about to hop on a ten-hour flight, let me just start by saying the finale was very uncool for that specific reason.
Well, it wasn’t a real plane, so you’re safe. [Laughs.]

Sian Brooke completely knocked it out of the park as Eurus Holmes. Since Eurus doesn’t actually appear in Doyle’s books, would you could walk me through how you chose to approach her character, and why you decided to introduce her this season?
We already had the relationship between Sherlock and Mycroft, which had been very useful for us, particularly with the fact that Mycroft is a bit colder and smarter than Sherlock Holmes. There’s always been rumors and theories about a third brother called Sherrinford. So we just wondered what would could do with his sister that would at least be different. And also, what if she would be extreme, and if she was even more heartless and even more intelligent than Mycroft. It would be the final stage of Sherlock learning to place some faith in his ability to make human connections. Obviously, the story of our series has been about Sherlock finding his way into that, starting to understand that all these things are not worthless, there are strengths to be gained from them. And, well, here we are. She illustrates that for him; she’s so much smarter than him, but so much more mad. She doesn’t have the balance of a human connection and of an understanding of basic empathy, until Sherlock reveals that, really, she’s still a child in the end. That he is, as his mom says to him, “Always a grown-up.” The thing about Sherlock Holmes is that he’s actually more grown up than both his brother and his sister. But it is that part of himself that he didn’t value — you know, his emotional side. Once you get to the main Sherlock Holmes stories in the middle of the run, Sherlock Holmes is a much more humane and wise figure than we’ve presented him to date, because the final stage is him learning the value of being like that and the peril of not being like that.

How would you define the dynamic between all three of the Holmes siblings? Because you’re absolutely right: Sherlock seems to be the emotional heart.
Well, they have extraordinary mental equipment is what they’ve got, an extraordinary mental acuity. They are extremely brave, the computer runs well. Both Mycroft and to a greater degree, Eurus, cannot see value in morality beyond the idea that it is the survival strategy of a pack animal. Sherlock Holmes can. He can actually feel and experience compassion. And he is quite a compassionate, passionate man underneath it all. But that’s always been the case for Sherlock Holmes. He is claiming to be the arch-intellectual, but we all know he’s a wise old man and a hero, too. Eurus reveals the extremes of where a great brain can go without a great heart to accompany it.

I think the most surprising development of the season is how deeply psychotic and disturbing Eurus is, which is shown through childhood flashbacks. Why is it important to not just have characters discuss their pasts, but to actually show it so vividly?
It’s not so much important as interesting. It was an interesting way to go with Sherlock Holmes, partly because it’s not what was done. In a way, Mycroft and Eurus are an alternative version of him; they are what he might’ve become in other circumstances. It was interesting to go there, rather than thinking it was important. It was a fun thing to do. We hadn’t seen it done before, a Holmes sister and a completely nuts one. It was fairly obvious that Mycroft and Sherlock were both a bit mad. It’s just ramping up the madness and saying she is this super-smart and super-insane one. It just seemed like fun, an interesting thing to do with it.

It was also very good fun to see Andrew Scott return as Moriarty in all of his Queen-dancing glory. How did you decide the he and Eurus would scheme together in Sherrinford all of those years ago?
We knew that we wanted Moriarty, after killing himself, to have set up a disaster for Sherlock Holmes. Making it his sister, barreling into the root of it, was an obvious way to go. Moriarty had to be dead at the end of the second series, so it’s nice to be able to give him an out. He’s arch-nemesis, he’s the one people look forward to and the one people miss, so we gave him one last look at it. It’s really no more than that.

I’m curious about your inspirations for Sherrinford. It reminded me of Saw a little bit. A friend of mine even said Eurus was Hannibal Lecter–esque. Did you take certain inspirations while crafting that unique world?
You’d have to ask Arwel [Wyn Jones] some of those questions because he’s actually the designer who made that. I’ve never seen Saw, though several people have mentioned it to me since the episode has gone out, and I haven’t seen any of those films. I’ve obviously seen Silence of the Lambs, and that’s where we got the idea, “Let’s have the glass wall there, let’s not have a glass wall there, that would be scary.” But, you know, we wanted to go the whole hog: Sherlock Holmes and Dr. Watson trapped inside a demented lab experiment, having to solve cases. It’s a little bit James Bond I guess; it’s a little bit horror movie.

I liked how action-packed it was. It was the closest thing Sherlock has had to a Bond episode.
But James Bond’s not really like that. There’s never been a James Bond where he’s trapped in one place for the entire time. [Laughs.] But, I guess so. In all the chatter, we’ve probably forgotten that both Sherlock Holmes and James Bond, in their literary originals, belong to the same genre of the smart British person going into battle against evil fiends. There’s quite a lot in common between the two, so they’re not completely different. Every single James Bond villain is basically Moriarty. They all are. They really talk like Moriarty. They all get their personal styles from Moriarty, so there’s commonalities there that we’re not frightened of.

For a man as sharp as Mycroft, I was pretty surprised that he let five minutes of unsupervised conversation occur between Moriarty and Eurus. What was his reasoning behind that decision, and would you agree that it was pretty reckless?
Well, the reasoning is the show, because after those five minutes she’s gonna anticipate the next terrorist attack. He made exchanges, he made use of Eurus’s monkey brain for the security of the nation. He thought, “Okay, five minutes, unsupervised. How much harm can she do?” I mean, he got it very badly wrong. He thought the island was secure. But you know, the mistake wasn’t that, so much as the mistake was letting the governor into the cell with Eurus to interview her. I think that’s where they went disastrously wrong. The governor is the one who makes the huge error, not Mycroft. It wasn’t smart, but he could not have anticipated that. Smart people make mistakes, you know. That’s not an uncommon fact.

You and Mark Gatiss have spoken out quite a bit about being unsure if this would be the last season of Sherlock. Why do you think this would be the right time to end the series? The final, non-cliffhanger montage definitely had a feeling of closure.
Well, it closes a chapter. It closes an idea. Dr. Watson is at last the brave widower and Sherlock Holmes is a wise old sage of Baker Street. They become, as we keep saying, the full Rathbone, which is why we actually have Rathbone Place up on screen as they come. Finally, we’ve done four series of Sherlock. It begins as a voyage, as a grand origin story, never the grown-up version. There is a proper sense of closure because that’s the end of stage one. Normally, we present Sherlock Holmes and Dr. Watson as in their 40s or 50s, and we started with them in their 30s, so in a way, it’s the end of their youth. If we came back again, if we did do another series in a few year’s time, then they would be, finally, the normal age for Sherlock Holmes and Dr. Watson, who were always presented as older. You would have them in the midstream of everything, as opposed to the beginning. So it can have a sense of closure in that, but, you know, that practically says they’re off to solve more crimes. It’s not closure; it’s closure of one idea. It’s them realizing that they are always going to be in their flats or grinds. That’s their future.

If the fifth season were to come to fruition, which of Doyle’s stories would you like to adapt?
Very, very few of the stories adapt very easily, because properly paced, you could get about 20 minutes out of a Sherlock story. We’ve done “The Hounds of Baskerville.” We’ve sort of done “The Sign of the Four” in different ways. The rest of them you can’t really adapt into movies. You have to come up with a new story involving elements of the short stories. Maybe taking a villain from one and combining them with the crime from another. There are loads of great sequences and great ideas. There aren’t feature-length ideas. That’s not what he was in the game of. So we did have to make up new stuff. [Laughs.] Mark’s always wanted to do “The Red-Headed League.” And I’ve done the ones I’ve always wanted to do: “The Dying Detective” and “Charles Augustus Milverton.” I’ve done both of those. There’s a ton of other stuff in there. There’s “The Engineer’s Thumb,” which is a slightly mad story that doesn’t have a proper ending. There’s an element of “The Greek Interpreter” that I think is really exciting, which hasn’t been done, because in our version of events Irene Adler is still out there. There’s always that. Now we know she and Sherlock actually still text each other. What would happen if they ever met again? There are those things we can do, but we simply have no idea whether we’ll be doing them or not.

This interview has been edited and condensed. 

Sherlock’s Steven Moffat Unpacks That Big Finale