Olivia Cooke on Returning to ‘Slow Horses,’ On-Set Injuries and That Shocking Finale Cliffhanger: ‘I Want a Showdown!’

SPOILER ALERT: The following article contains plot details from “Judgement Day,” the Season 6 finale of “Slow Horses,” now streaming on Apple TV.

“Hello, River.”

Those two fateful words announced Olivia Cooke’s return to “Slow Horses” after a four-season absence from the Apple TV spy drama. Cooke’s MI5 agent Sidonie Baker made a brief but memorable appearance in Season 1, forging a flirty chemistry with inept spy River Cartwright (Jack Lowden) before dying from a gunshot wound to the head — or so River thinks.

Sid’s resurrection wasn’t a total surprise to “Slow Horses” fans: the character comes back in author Mick Herron’s namesake novels, and Apple made little secret of her storyline in the advance promotion for Season 6, which concluded this week. But enough has changed since Cooke’s first stint on the show to make her re-joining the ensemble an event in itself. First, “Slow Horses” itself grew into a hit, with a raft of Emmy nominations (plus a few wins) and an avid audience to show for it; meanwhile, Cooke’s own career has taken off thanks to a leading role in the fantasy epic “House of the Dragon.”

That star power is on full display throughout Season 6, which sees Sid and River on the run from Russian sleeper agents over one incredibly stressful night. Icy MI5 head Diana Taverner (Kristin Scott Thomas) — who once planted her favored mentee Sid in Slough House, a kind of purgatory for screwup agents, as her eyes and ears — used Sid’s identity as a cover for an off-the-books assassination on Russian soil, and now the Kremlin is out for revenge. All of the Slow Horses (a not-so-affectionate nickname for Slough House employees), past and present, are targeted, and Sid goes to River for help.

The pair’s winding route from River’s late grandfather’s country house to Diana’s beach retreat, which Slough House head Jackson Lamb (Gary Oldman) cheekily appropriates as a safe house, is a winding one. Sid and River barrel through a barn, an arcade, and finally an airstrip where Sid is nearly shipped off to Russia as an unwilling sacrifice on their way to a long-awaited first kiss. This being “Slow Horses,” though, their triumph is short-lived: River helps Sid escape and nearly reunites with her at his grandfather’s house, but collapses from an apparent poisoning in the kitchen while she showers upstairs in the season’s final moments.

“River has kept her going, in a sense,” Cooke says now of the star-crossed lovers. Speaking with Variety from the U.K., the actor shared what it’s been like to dive back into the deep end with over a month of night shoots, from an unlikely injury to her collaboration with Lowden.

Your character comes back in the books, but I’m sure when you were first filming, it wasn’t certain we would get to this point. Did you know there was a chance you would return?

We shot [Season 1] towards the end of 2020. It was one of the first projects back after lockdown. I don’t know if the book had come out yet where I make my return, but there were whispers of it. I don’t know. It was such an uncertain time, and it was the first series as well. It just felt, like, an eon away. Who would have thought it would have gone on for so long?

But they kept me abreast of all the things that were going on story-wise over the years. I’ve been admiring from afar, and biding my time. I’d get messages from Jack being like, “You’re coming back!” and I’m like, “Fantastic, when?” And no one could really give me a clear answer. I remember seeing Saskia [Reeves], who plays Standish, at the theater, and she was like, “You coming back? And I was like, “I would love to. I just don’t know. I have no clear answers.” We were all just in a cloud of mystery for a bit.

During that time, what was it like for you to watch this show that you were once a part of, but weren’t for a while?

Really fun, but tinged with jealousy. Because I experienced it, you know? Even though we were all masked up and socially distanced from each other and the crew when we weren’t in the scenes, it was such a fun, charmed environment that I was just desperate to get back to. And it’s so palpable — watching Season 2, 3, 4, 5 — just how much fun everyone’s having on set. It really just glows through the screen. So it’s always bittersweet. I love watching them, and I love seeing how good all my friends are, but I’m also incredibly jealous as well that they’re all having fun without me.

I imagine that, as an actor, having to return to a character that you left behind is a challenge. Is there any work or preparation that you did to reassume this role?

I had to remind myself how I sounded, like what accent I was doing. I had to check back in with that. But also, it was quite nice, because Sid is coming back in a very jaded way. Both her and River have grown up mentally in the interim. They’ve been away from each other, and the events that led up to Sid’s disappearance were really traumatic. So I had a bit of rope to not honor the more jovial, happy-go-lucky version of Sid that we saw in the first season.

We don’t get a lot of detail about what the last three years were like for Sid, because there’s a lot happening in the present. Do you have your own sense of that backstory?

I was asking Adam [Randall], the director, a lot about that. We thought that she’d been in a rehabilitation facility that is kept under wraps by MI5. It’s for ex-spies, ex-Joes, and she’s had to learn how to walk and talk again, basically. It’s been an incredibly lonely process that has, I think, cauterized a part of her, in some non-descript location. We were thinking maybe Cumbria, if you know of it. Sort of like the the Vermont of England — very green, but you can get lost in the landscape. I think it’s just been a very long road of recovery, and also of introspection for her.

You mentioned the FOMO earlier, so I’m also interested in what it was like for you to actually step back into the set. I’m sure the non-COVID version of the show feels different.

Well, it was pretty much the same crew, and also it’s a lot of the same crew that does “House the Dragon” as well. Like on off seasons with “Slow Horses,” there’ll be people that do dailies and things on “House the Dragon” and vice versa. So I know everyone quite well. It was really lovely. It’s like the best high-school reunion possible. As if you went to a high school and everyone’s your friend, which I don’t think is a reality for anyone. It was just really joyous, even though we shot it right before the December break, in the middle of winter, all night shoots for five weeks, because they truncated all my stuff.

I was going to ask about that too — the nocturnal nature of it.

I sort of loved it. Adam has this energy about him where it sometimes feels a bit run-and-gun, which I love. And if a particular stunt doesn’t work out, or when we get to the set, the material doesn’t quite match with how the workings of the set are, he’s like, “All right, let’s figure this out. Let’s improvise.” It’s so lovely for me as an actor, too, because I feel like there’s a real energy to that. It did just feel like we were making an independent film for five weeks, because the majority of my stuff was just with Jack and Adam.

Your storyline this season is almost entirely two-handers with Jack. What’s he like as a collaborator?

He’s my favorite type of man, which is a very silly man. It’s wonderful acting with him, because he just chucks you a curveball, and then you try and grab it. It makes me feel like I’m stretching my mind in ways that I don’t usually get to do with doing this job. And he’s so fun! He wears it very lightly. He’s such a good actor. He’s very funny, and doesn’t take it too seriously either. Which I love, because sometimes, with other male actors, they take it very serious.

In addition to the night shoots of it all, there’s a lot of action on the show. Were there any particularly tricky sequences?

There were, but the one thing that actually did me in was just running. There’s a lot of scenes where I’m just running alongside Jack, who is twice as tall as me, and my 5-foot-5 arse is just desperately trying to keep up. It was like 2 in the morning, and we were being chased by a Land Rover. You’re stretching, you’re trying to keep warm, but I think I’d been a bit static for too long, and we set off and I just felt both thighs go snap. I pulled both of my quad muscles really, really badly. For two weeks, I was hobbling around. Just from a run! But then I thought to myself, “When was the last time I got chased?” Since I was like 12!

You’ve gotta have your people page Tom Cruise’s people and be like, “How do I run onscreen?”

Literally! Climbing fences, shooting, being on the back of a moving truck while there’s all these fire and gas canisters going off — but just running really, really did me in.

I was curious about the barn interlude. I can’t imagine being in a barn full of smelly cattle for hours on end.

The thing is, you can’t really do anything about it. You just have full acceptance 20 minutes in, and get used to it. But we were moving around in and amongst a lot of cows, which was quite scary, actually. They’re really big, and they can kick! I was just remembering old horror stories about people being killed by a stampede of cows, just walking across a field, and I was like, “I’m so close to them!” But it was fine, no one died.

The other action moment that really stood out for me is when you’re on the truck and kicking the oil drum, and it explodes. That looks tricky.

That was a crazy, crazy sequence, just trying to dispose of the Russian agents who had come to collect me, and running and jumping on the truck and then Jack just doing donuts around a plane. And you know, Jack does all his own driving stunts! It was very impressive. That was so fun, so cathartic. I love kicking things, and then having this big old stunt where it blows up — it felt like our own Jason Bourne moment.

In addition to the action, Sid has a whole emotional trajectory. She seems reluctant for most of the season to accept just how ruthless Diana Taverner has been with her. Why do you think Sid clings to that so much?

Because Sid thinks, ultimately, that Diana saved her by getting her out of the hospital, by putting her into some specialized rehab. I think she does genuinely think that she’s only here, she’s only alive because of Diana Taverner. So there’s, I think, some real Stockholm syndrome going on with her. Also, she’s only ever experienced one side of Diana, which has always been very collegial. I think Sid has liked how impressed Diana’s been with her in the past, and hasn’t seen this steely, ruthless side to her that everyone else has experienced.

How do you think she feels once she speaks to Diana on the phone and can’t deny what’s happening anymore?

It’s incredibly clarifying, and almost a relief to know that this is the truth — the cold, hard truth. This person thinks that I’m dispensable, that I’m a pawn in whatever game that Diana is trying to play. Knowing that her life could be forfeit so easily — I think through the shock of it — she’s almost giddy. Like, “Oh shit, this is all so obvious to me now.” She kind of has nothing to lose from that point on, because it’s like, “Well, I’m gonna die regardless, going on this plane with these people, so I might as well just try to get out of here.”

You and Jack have great comedic chemistry. But because it’s a spy show, and especially because of what happens at the end of the season, you’re also meant to be questioning everyone’s motivations all the time. Do you have your own read on how Sid feels about River, and how sincere those emotions are?

I think despite herself, especially in the first season, her feelings for River have really caught her off guard. I think there’s a genuine connection — this tiny little pinhole of light. River has kept her going, in a sense. Even if they didn’t really get to know each other that well, whatever she’s projected onto River, and the reason why she seeks him out, is because there is something there that has been fizzing for a very, very long time. Especially in the absence. I think those things only get bigger when they’re unresolved, and she’s been so adrift for so long.

I was devastated by the cliffhanger. I can’t believe I have to wait an entire year to find out what happened.

I know, I know!

Are you someone who reads the books, both to find out what happens with your character and maybe see what happens down the road?

In the interim, I hadn’t read the books, but I read for the season that is about to come out, just because I was like, “Surely I’m coming back!” But [Mick Herron] is such a brilliant writer, and he’s so acerbic, and sometimes it’s not faithfully adapted, but they’re really good companions to each other.

Obviously, you can’t tell me anything too specific about the future on the show, but the one thing I was very bummed out by is that we actually don’t get you and Diana face to face in this season. Do we think that might happen in the future?

I hope so! I want a showdown. With Kristin Scott Thomas, I think she’s a wonderful actor. I could watch her endlessly. So selfishly, I want that for me, for my dear diary. But yes, it feels like that is a very cathartic thing that must happen.

You’re heading into the final season of “House of the Dragon.” Do you have any goals or plans for the other side of that, when there’s gonna be some free time in your schedule?

God, I want to do so much. I really want to do theater. I really, really desperately want to do theater. I am trying. I’ve been trying for years to find the right thing, and maybe I’m just being a bit too picky, and I just need to do it.

But theater, and I’d like to be on the other side of the camera a bit more. I’ve been doing this for 14 years now, which doesn’t sound like a lot, but it feels like a lot. And I think I want to find new avenues to go down because I’m getting a bit sick of myself.