Olivia Cooke Explains Her Surprise Return to Slow Horses

Spoilers below.

She’s back! It was only in the second episode of Slow Horses that Sidonie Baker, an accomplished MI6 agent played by Olivia Cooke, was shot in the head and presumably killed. But five seasons later, it seems like that wasn’t the full story: Sid appears—alive—in the trailer for season 6, which will premiere on September 16.

Cooke, who also stars on House of the Dragon and the 2025 miniseries The Girlfriend, was thrilled to be back on the set of the celebrated Apple TV series, which follows a group of disgraced and demoted MI5 agents. Cooke spoke with ELLE about her return, how the Slow Horses set is different from House of the Dragon’s, and why the show’s humor resonates so deeply with her.

Did you know that your character would return, or was it a surprise when you got the call?

It was six years ago when we filmed the first two [seasons], and I think there were murmurings of, “You’re going to come back,” and then the Slough House book came out, and I was in it. But you never know. It’s never guaranteed. I didn’t know how faithfully they were going to keep to the books, but there were various availability checks over the years. I was like, “Okay, they’re still hoping to use me at some point.” I was biding my time because I was desperate to work. I’ve been such a fan in the interim.

Were you excited when you found out you were coming back for season 6?

I was champing at the bit to get back on set. I filmed it straight after I finished shooting The Girlfriend, at the end of 2024. All my stuff was truncated to pretty much five weeks of night shoots just before the Christmas break, so it felt like we were making our own little independent movie. It was really joyful, even though we’re being chased and shot at. This is a job that feels like adult play, especially coming off the back of shooting The Girlfriend, which was very intense, and playing Alicent on House of the Dragon—does she have teeth? Who knows, because she never smiles. It was really nice to be sort of sarcastic and upbeat and to just play and stretch my arms a bit.

Did you have to keep your return a secret? Fans of the book knew your character wasn’t dead, but it’s a surprise to many of the show’s fans.

I’ve told quite a lot of people. I keep forgetting because people are like, “Oh, what are you up to?” I’m like, “Oh, I’m just shooting Slow Horses.” They were like, “You were shot in the head, you died.” And I’m like, “Thanks. Thanks for that.” And I’m like, “Shit.” So yeah, technically a secret. I find it very hard to keep secrets about myself.

House of the Dragon is probably very secretive, too.

You’re sent the script and it’s like Fort Knox trying to get on the database where the scripts exist. Although it seems like an episode leaks every single week, so I don’t know why [the cast] has to be so under lock and key about the scripts when we’re filming, because the leaks never come from us. But yes, it’s very, very
secretive.

How do you feel about filming the last season of House of the Dragon?

I’ve known that it’s going to be four seasons total the whole time, so this feels like a very natural end. I’m sure it’ll hit me once we’ve finished shooting the season, but we’re still a ways off [from] being on set for the last season. We won’t go back ‘til next year. I’m sure I’ll feel lots of conflicting emotions. I hate endings anyway, and you have a lot of endings in this career, and you have to say goodbye to a lot of peculiar families that you make along the way a lot of the time. It’s going to be really, really strange not to see those people and not to have this intensity every couple of years together.

Does Slow Horses have a different tone than other projects you’ve been on? It’s obviously different than House of the Dragon.

Will Smith, the writer on the first five seasons, [was a writer and actor on] The Thick of It, so it’s definitely got this incredibly acid sarcasm to it that’s said with humor, but if you took it literally, it would wound. I love that stuff. That’s how I grew up talking to my friends and family. That’s how they still speak to me. It’s endearment, really. I’ve never been able to work in that space apart from on this job, something where the voice feels so quintessentially British. It’s really exciting because this is a language that I speak so easily. It’s a bit harder to get into the tone and feel of a fantasy medieval world and the language around that, [while] also being a queen at core. That’s something that I have to research and learn, but the humor of Slow Horses comes, and hopefully it translates, very naturally to me.

This interview has been edited and condensed for clarity.