Finally, Olivia Cooke is running with Slow Horses again.

At long last, Olivia Cooke is coming home.

With the arrival of the sixth season of Slow Horses, Apple TV returns the viewer to the dysfunctional team of British intelligence agents of Slough House, a dumping ground for MI5 undesirables. Led by the irritable Jackson Lamb (Gary Oldman), this team of spies are filled with those who have marred their careers and have nowhere else to go.

Now, the team from Slough House finds themselves in more trouble than ever as they are slowly being erased. Former members are dying mysteriously and MI5 seems to have disavowed their team. This adds a darker element to the series, forcing the team out of their home base and setting them on the run.

Adding to the drama is the return of Olivia Cooke’s much beloved character, Sid who has finally been resurrected from the grave. Though her time on the series only previously lasted a few episodes, her character has cast a long shadow on the team at Slough House, especially River (Jack Lowden). Although fans hoped for her return, it was never a certainty. Even so, Cooke always remained optimistic about the possibility.

“I don’t know if I’ve made this up in my head, but this is the story that I’m sticking with,” Cooke laughs. “I knew that she was going to come back. I didn’t know if they were going to be faithful to the books, but there were various availability checks over the years. So, in 2022, there’d be a call, ‘what’s Olivia’s year looking like?’ 2023, another call. “What’s Olivia up to? What’s the year looking like?’ And at this point, it felt like I was in a really bad relationship with someone who didn’t really like me but was just dangling this little carrot. Then, at the beginning of 2024, they were like, ‘What’s Olivia’s year looking like? We’d love to use her at the end of the year for season six. This is the outline.’

“And, reading it, it felt so wonderful and perfect in terms of coming back at this stage of my life and all the questions that you have about Sid as well. It just, it felt really wonderful and triumphant.”

Despite her anticipation, it’s also hard to go home again. Having been away from the team for four years, one might expect that her return might have been difficult on Cooke as she and her fellow castmates have grown and changed as performers. However, Cooke believes that that time away has helped has an actress – and Sid as a character.

“Well, it was all really helpful. I’d been away from that set for four years and everyone’s in different places in their lives,” she points out. “I started that job when I was 26, and then I was coming back to it in my 30s. So, lots have changed, and I think it was really helpful to help have that imbue Sid’s relationship with River. I think she has sort of immortalized and mythologized what that was in her head, because, in the interim, she’s been in rehabilitation.”

“But she’s also not allowed to have any real human interactions because officially she’s dead or doesn’t exist. So, I think she’s been incredibly lonely, and her trying to pick up with River where they left off was just never going to happen and never going to work. And so, there is a lot of bracken and debris to get through in order to find that sort of sizzle that they used to have.”

However, in addition to being marred by the scars of their past, her relationship with River also carries a deep sense of grief. As River deals with the loss of his grandfather, the return of Sid simply doesn’t feel real to him, a journey that isn’t lost on the actress or her character.

“Yeah, [Sid] sort of has to keep trying to keep convincing River that this is real,” Cooke suggests. “And I think when you’re in that grief state, your reality does feel really flimsy. So, I think she has to convince River quite a lot that she’s being her authentic self, and she’s not going to abandon him again. She abandoned him once and his grandfather’s dead, and there’s a lot of pain happening. You know, this season takes place over maybe two nights and three days, so we see him bury his grandfather, and then, that night, he sees Sid, and then they’re on the run. So, it all happens in such quick succession, and there’s a lot of interpersonal dynamics that needed to be sorted out.”

And the intensity of those circumstances have definitely left a mark on Sid. Asked if she believes that Sid still wants to be a spy, Cooke is pensive, pointing out that her character has a lot baggage to deal with.

“I think she still has a very warped and positive perception of who Diana Tavanagh is that is muddying her wants and her needs right now,” she says. “I don’t think she’s had a lot of autonomy over the last few years, but her only beacon of hope is what Diana Tavanagh has been telling her, which hasn’t been truthful at all. So, I don’t know. I think she’s very good at being a spy. She’s very competent but she also has nearly died and she’s had a lot of contemplation time over the years.”

And Cooke can relate. Having been away from Slough House for some time, she’s been working in a completely different world as Alicent Hightower on House of the Dragon. Although, despite their differences, Cooke believes that Sid allows her to access a part of her personality that Alicent does not.

“Yeah, they’re so tonally different, Alicent and Sid,” Cooke responds. “I think Sid is much closer to who I am as a person. I’m able to tap into my sarcasm and humor a lot more. I think that’s both very emotionally repressed, so there’s that. But that’s, I think, is just being British. I don’t think that’s anything to do with character. But yeah, it’s so completely different. I do think even since season one, we’ve been encouraged to dial up the things in us that are most in alignment with the characters and the tone of the series and that sort of continued.”

The premiere of Slow Horses is available on stream on Apple TV on Wednesday, September 16^th, 2026 with new episodes streaming weekly afterwards.